This campaign isn't exactly the "normal" campaign you'd usually play. You're expected to pretend to be in a movie setting similar to Zombieland x 28 Days Later (kinda redundant context-wise given this is Left 4 Dead). The infected spawn is a bit iffy (especially the finale), the neurotoxin tunnels are a hassle, the finale has a bit of broken hitboxes, supplies can be scarce (please bring a melee weapon), but believe me, this campaign is nuts. You'll be sent into multiple highly-detailed environments that will, not once, look empty all the while having other NPCs accompanying you. The voice acting is legitimately amazing; a rare thing to have in custom campaigns. The environmental storytelling makes sense and really puts you in the scene. The layout won't put you into endless red herrings - they're as clear as they can be but still enough to contribute to the overall environment than being obvious directional props.

I swear, this whole campaign feels and plays like a movie more than Valve's original campaigns. Matter of fact, don't play it with just four people, go nuts - play it with 8 survivors, 10, or even 20.

Overkill stood the test of time and even a decade later, it still kicks ass. I highly recommend playing it, even on Easy, just for the experience alone.