It's worth and it should be on sale. You can check steamdb for more info about sales.

Depends on you if your time is worth it.

I literally just now completed the last gun after playing on and off for years, and just recently (last 2 months) grinding out the remainders. Put $0 into the game.

It can be done either way, it's up to you.

Once I saw it back in 2018, I immediately bought it (waiting for a sale is wiser), because that's exactly what I wanted — a virtual reference for three-dimensional gun models* at your fingertips (provided you have your PC/laptop with you and an active internet connection, since a truly offline mode isn't a thing with this game). If that is what you want right from the start without wasting time on "minigames" (which aren't very entertaining, to be honest), then the answer is unequivocally "yes".

The simplest way to put it is to think of this as a tool — it can be used for entertainment, it can be used for education (albeit, quite limited so*), and, of course, it can be misused (i.e. naively trusting everything), as almost any other tool known to man can be used or misused in different situations, depending on one's needs.

*There is a big asterisk here, which one usually learns after having hands-on experience with firearms IRL. Some of the models in the game are popular, some are quite rare, and some are potentially misleading, because the models are in certain calibers (certain models of IRL weapons, especially contemporary ones, can often operate in different calibers and this changes pressure values, part dimensions, required materials, sometimes the manufacturer names different models based on the caliber of the firearm (the clearest example is the company, named Glock), etc. These things, including the most important things around firearms — safety rules, are not things that one can learn from this game, unfortunately.