Really enjoy this game but it's very much like L4D 1/2. Nothing wrong with that, it just has every single "special" monster that L4D has, just renamed.
Still, amazing game. I love the forge and how you can upgrade your weapons as well as the fact that you can switch between characters whenever you want (L4D again) ;)

Definitely buy it if you like L4D. 100%

Just want to note that it's crashed on me a few times and there are a few glitches in game now and again. Also, I have quite a powerful PC and it surprises me that I get fairly low FPS on high graphics, around 40fps on average.