A pleasant surprise of a game. Killing Floor is much more than meets the eye.
The core gameplay is all about killing different types of mutated enemies in coop with up to 6 others. The game's class system tends to be actually very fun, and it rewards long time players with some pretty nice bonuses for being high level. Sometimes the presence of level 6 players in a game can ruin it by making it impossible to get any kills, but generally the rewards for leveling up you classes are balanced and fun. There's a lot of room for creativity in ways to defeat the waves of mutants, and a door welding tool makes for great fun in doing so. Overall the gameplay is very solid.
The graphics are a bit dated, but it still manages to be really scary with the technology available. Based on the Unreal 2 engine, the game looks very much like a mod, but it is mostly visually attractive. Some of the levels can be entirely too dark and you have to really memorize their layouts before you can excel.
I'd recommend this game to any fans of shooters (coop or otherwise) looking for something old-school but fresh.