I'm going to go ahead and review this despite not being really close to a level of completion I am with other games I review. This is due to the sheer scale of the game. TL;DR at bottom.
Ask me any questions you like in the comments! I'll try to respond quickly.

Like many others have said, this game is phenomenal. It is the co-op PvE game that ruins all other co-op PvE games.
For background, I've levelled up one class, the gunner, to 25 and then promoted. He's pretty much all I've played. I just got started with Deep Dives and play at a level of Haz 4 solo, Haz 5 with friends. I also don't mean to imply I have any sort of deep knowledge about the game. This is important, because from what I've seen in 45 hours of game time, I have a LOT to learn.

The Gunner is the first class you're given when you start the game. It's the "tutorial" class and meant to be simplistic while teaching you about the game. The closest comparison I have is that he's the Heavy from TF2 but better.
The Gunner has 3 different primary weapon options, 3 different secondary options, and 3 different grenades to choose from; and they're all really useful, unlike many of Heavy's options. Like most of the classes from TF2, you also can note that there is a significant difference between a new Gunner and a veteran Gunner. Knowing where to position yourself, when to use what weapon, when to carry which weapons into a mission, and how to budget your ammo is just scratching the surface of how deep this "tutorial" class can be.
With DRG, all cave systems are procedural. This means instead of knowing which geometry you can duck behind in TF2 while still being able to fire with Heavy is an important skill, all knowledge pertains to your class and how to perform in new and interesting situations. This further deepens with multiple mission types ranging from Mining Expeditions, to Point Extraction, to On-Site Refining, etc. In these missions you're ranging from being able to be completely mobile to having to focus around a singular point to having to stay with an objective and keep it maintained.
Beyond that, there's also different biomes with different cave generation gimmicks and terrain features. And even game modifiers that may be beneficial or detrimental!.
For the purposes of this review, I won't get into all of the depth that I've found out about the Gunner, but needless to say as a fan of hidden techniques and knowledge in video games, I am very satisfied with the amount of depth I've gotten.

Back to progression, there's a perk system in here as well. It frankly really needs a rework. I find a portion of the perks to be useless, and it seems the community agrees with me for the most part on this one. However, enough of them are good options that you can easily get whatever perks you need to help fill out a niche or support a weakness you have, whether its class-based or skill-based.
Promotion works a bit like CoD's prestige, except there's no downsides; nothing resets, you just unlock more things.

There's an upgrade tree for each weapon where you pick and choose certain upgrades. Once you buy them, you don't have to rebuy them again as opposed to PD2's system.
There's also Overclocks, which help change your weapon in minor or major ways. Pretty hard and slow to earn, but can be well worth it.

To touch on difficulty, there are 5 Hazard Levels, with various modifiers that can stack on top of those. Hazard 1 and 2 should be combined into one, Hazard 3 is good for new players, Hazard 4 is a happy medium, and Hazard 5 is usually a really great difficulty option. Sometimes it can be way too hard on certain types of missions though, but this is likely just me being unskilled at this game.
Different difficulty options affect your rewards which include credits, XP, and minerals, all of which are important in buying items and upgrades. You may want to bump a Haz 4 mission to Haz 5 because it has some minerals you need for an upgrade. It's a very natural push where you are incentivized to push your limits.

Customization is a huge thing in DRG. There's an absurd amount of cosmetics. 99% of them are unlocked by playing. Sometimes there's certain events in missions that you can randomly find that unlock you cosmetics. There's currently two paid DLC cosmetic packs, but the ones in-game are very similar in terms of quality.
There's skins for guns, visual addons for guns, head, chest, hair color, skin color, armor color cosmetics, etc.
Loadouts are also very important. You can have up to 5 loadouts per character with different cosmetics, weapons, perks, etc similar to Payday 2. You usually want to have a few, as certain missions will be easier with certain loadouts.

Options; there's a lot. The biggest thing I want to touch on is that you can customize your hud pretty freely. You can enable, disable, or dynamically display certain elements. It's really fun to see how much you can give up to help immerse you in DRG or make things harder. However, I wish there were more options for people like me who like to play without a hud, as we have to rely on mods to substitute where default lacks.

The modding scene is one of the best organized I've seen in any game. It's officially supported, with mods fitting into certain categories with restrictions based on the changes the mods make.
You can install mods on a pseudo-Steam workshop page, and it's very easy to do so. There's one or two mods I think should be included in the base game, but I don't think you need any mods to heavily enjoy DRG.

The community for this game is pretty awesome. There's a lot of catchphrases that helps foster familiarity and a bond between randoms, and everyone I've encountered playing online is super nice or is just new and hasn't learned dwarven etiquette yet.
I have limited experience with the community though, since I mostly play with friends. Regardless, it feels like a step above Payday 2's community.

Visuals are pretty simple, Everything is made up of simple polygons and triangles, resulting in near-zero reliance on your graphics card. However, it makes up for it through good use of lighting and effects. It does what it needs to and does it well.

Warning: This game is incredibly CPU intensive. Make sure you have a good processor. Also there are some bugs that may cause low performance.

Sound design is really important for this game; the caves you're in are going to be very dark, only lit up by rechargeable flares all classes can toss around. It's important to have clear sound design so players can use sound as another method of information gathering.
For specific heavy-duty mobs, they have distinct sound cues that allow you to clue in on what's coming towards you. This key information will allow you to prepare for certain scenarios without even seeing what's happening. I can't tell you how many times I've been able to avoid bad situations by hearing something before it appeared and adjusting accordingly.

As for music, it's okay. I haven't really listened to much of it, as it didn't blow me away. It's nothing compared to Simon Viklund's Payday 2 soundtrack.

Best PvE game on the market, with excellent sound design, gameplay, depth, and procedural generation. Intelligent and phenomenal modding scene with official developer support, and in-depth customization and UI options.
Developers could add a little more options in terms of UI customization, and performance can be pretty poor.
Overall, there is no competition. Deep Rock is King. Buy it, and rock and stone!

I'm going to go ahead and review this despite not being really close to a level of completion I am with other games I review. This is due to the sheer scale of the game. TL;DR at bottom. Ask me any questions you like in the comments! I'll try to respond quickly. Like many others have said, this game is [b]phenomenal[/b]. It is [b][i]the co-op PvE game that ruins all other co-op PvE games[/i][/b]. For background, I've levelled up one class, the gunner, to 25 and then promoted. He's pretty much all I've played. I just got started with Deep Dives and play at a level of Haz 4 solo, Haz 5 with friends. I also don't mean to imply I have any sort of deep knowledge about the game. This is important, because from what I've seen in 45 hours of game time, I have a LOT to learn. [h2]Gameplay (with the Gunner) and Progression[/h2] The Gunner is the first class you're given when you start the game. It's the "tutorial" class and meant to be simplistic while teaching you about the game. The closest comparison I have is that he's the Heavy from TF2 but better. The Gunner has 3 different primary weapon options, 3 different secondary options, and 3 different grenades to choose from; and they're all really useful, unlike many of Heavy's options. Like most of the classes from TF2, you also can note that there is a significant difference between a new Gunner and a veteran Gunner. Knowing where to position yourself, when to use what weapon, when to carry which weapons into a mission, and how to budget your ammo is just scratching the surface of how deep this "tutorial" class can be. With DRG, [b]all cave systems are procedural[/b]. This means instead of knowing which geometry you can duck behind in TF2 while still being able to fire with Heavy is an important skill, all knowledge pertains to your class and how to perform in new and interesting situations. This further deepens with [b]multiple mission types ranging from Mining Expeditions, to Point Extraction, to On-Site Refining, etc.[/b] In these missions you're ranging from being able to be completely mobile to having to focus around a singular point to having to stay with an objective and keep it maintained. Beyond that, there's also [b]different biomes with different cave generation gimmicks and terrain features. And even game modifiers that may be beneficial or detrimental![/b]. For the purposes of this review, I won't get into all of the depth that I've found out about the Gunner, but needless to say as a fan of hidden techniques and knowledge in video games, I am very satisfied with the amount of depth I've gotten. Back to progression, there's a perk system in here as well. It frankly really needs a rework. I find a portion of the perks to be useless, and it seems the community agrees with me for the most part on this one. However, enough of them are good options that you can easily get whatever perks you need to help fill out a niche or support a weakness you have, whether its class-based or skill-based. Promotion works a bit like CoD's prestige, except there's no downsides; nothing resets, you just unlock more things. There's an upgrade tree for each weapon where you pick and choose certain upgrades. Once you buy them, you don't have to rebuy them again as opposed to PD2's system. There's also Overclocks, which help change your weapon in minor or major ways. Pretty hard and slow to earn, but can be well worth it. To touch on difficulty, there are 5 Hazard Levels, with various modifiers that can stack on top of those. Hazard 1 and 2 should be combined into one, Hazard 3 is good for new players, Hazard 4 is a happy medium, and Hazard 5 is usually a really great difficulty option. Sometimes it can be way too hard on certain types of missions though, but this is likely just me being unskilled at this game. Different difficulty options affect your rewards which include credits, XP, and minerals, all of which are important in buying items and upgrades. You may want to bump a Haz 4 mission to Haz 5 because it has some minerals you need for an upgrade. It's a very natural push where you are incentivized to push your limits. [h2]Customization and Loadouts[/h2] Customization is a huge thing in DRG. There's an absurd amount of cosmetics. 99% of them are unlocked by playing. Sometimes there's certain events in missions that you can randomly find that unlock you cosmetics. There's currently two paid DLC cosmetic packs, but the ones in-game are very similar in terms of quality. There's skins for guns, visual addons for guns, head, chest, hair color, skin color, armor color cosmetics, etc. Loadouts are also very important. You can have up to 5 loadouts per character with different cosmetics, weapons, perks, etc similar to Payday 2. You usually want to have a few, as certain missions will be easier with certain loadouts. [h2]Options and Modding[/h2] Options; there's a lot. The biggest thing I want to touch on is that you can customize your hud pretty freely. You can enable, disable, or dynamically display certain elements. It's really fun to see how much you can give up to help immerse you in DRG or make things harder. However, I wish there were more options for people like me who like to play without a hud, as we have to rely on mods to substitute where default lacks. The modding scene is one of the best organized I've seen in any game. It's officially supported, with mods fitting into certain categories with restrictions based on the changes the mods make. You can install mods on a pseudo-Steam workshop page, and it's very easy to do so. There's one or two mods I think should be included in the base game, but I don't think you need any mods to heavily enjoy DRG. [h2]Community[/h2] The community for this game is pretty awesome. There's a lot of catchphrases that helps foster familiarity and a bond between randoms, and everyone I've encountered playing online is super nice or is just new and hasn't learned dwarven etiquette yet. I have limited experience with the community though, since I mostly play with friends. Regardless, it feels like a step above Payday 2's community. [h2]Visuals and Performance (WARNING)[/h2] Visuals are pretty simple, Everything is made up of simple polygons and triangles, resulting in near-zero reliance on your graphics card. However, it makes up for it through good use of lighting and effects. It does what it needs to and does it well. Warning: This game is incredibly CPU intensive. Make sure you have a good processor. Also there are some bugs that may cause low performance. [h2]Sound Design and Music[/h2] Sound design is really important for this game; the caves you're in are going to be very dark, only lit up by rechargeable flares all classes can toss around. It's important to have clear sound design so players can use sound as another method of information gathering. For specific heavy-duty mobs, they have distinct sound cues that allow you to clue in on what's coming towards you. This key information will allow you to prepare for certain scenarios without even seeing what's happening. I can't tell you how many times I've been able to avoid bad situations by hearing something before it appeared and adjusting accordingly. As for music, it's okay. I haven't really listened to much of it, as it didn't blow me away. It's nothing compared to Simon Viklund's Payday 2 soundtrack. [h1]TL;DR[/h1] Best PvE game on the market, with excellent sound design, gameplay, depth, and procedural generation. Intelligent and phenomenal modding scene with official developer support, and in-depth customization and UI options. Developers could add a little more options in terms of UI customization, and performance can be pretty poor. Overall, there is no competition. Deep Rock is King. Buy it, and rock and stone!