The premise of Killing Floor is thinner than thin. Some biotech company has been busy conducting genetic experiments for military purposes. These predictably go wrong, and now mutated, deadly, and very hostile Specimens are running all around London slaughtering and devouring people. You are a member of a British Special Forces team sent in to deal with this threat by using your advanced medical training to test the effects of bullets - on them. All right, maybe not so much that last part.
It's a very bare-bones premise, and it has no real hook to it - nothing like "You're the last four surviving humans in the zombie-infested city, you have to escape." Killing the Specimens isn't something you do en route to the goal, it is the goal. Your helicopter drops you off, and you're just going to have to kill everything that moves that isn't on your team. Games of Killing Floor consist of increasingly larger and more difficult waves of Specimens, culminating in a final boss fight, after which congratulations, you've won!
True, it's hardly reinventing the wheel, here. But to Killing Floor's credit, what it does, it does fairly well. The half-dozen levels in the game are all your standard horror fare - deserted farmhouse at night, the eerily abandoned streets of London, a dilapidated Victorian manor, etc. - and the game does a very good job at providing the proper atmosphere to get under your skin and creep you out. There's no such thing as a completely defensible position in Killing Floor: There are always multiple routes of entry to any given space, and even if you weld doors shut you'll always be having to check your back.
That's why you bring friends, of course. It's possible to play games of Killing Floor solo, but the game was clearly designed to be played cooperatively. Games can range from as few as two people to as many as thirty, with the sizes of the waves increasing relative to the number of players (why, is that a wave of over a thousand Specimens? I do believe it is!) Though it's wisest to stick together, the maps are fairly large, so it can be easy to get separated from your squadmates - a recipe for trouble.
The enemy Specimens eager to devour your succulent flesh supply a rather wide variety of foes to battle, from the fairly weak Clot to the extremely dangerous Flesh-Pound (who becomes enraged when shot with small-arms fire). Combine that with the handful of Perks you can select for your character, and you've got a game that still manages to feel fresh after you've been playing it for a while.
On the whole, though, Killing Floor feels somewhat inconsistent. Playing it cooperatively is entertaining, sure, and it can definitely be intense when you're in the thick of combat and frantically firing at a swarm of oncoming enemies. But that's simple run-and-gun action; we've seen that before. Killing Floor is at its best when you're playing it solo - when it's just you, your gun, and the slithering sounds of something lurking in the dark - and it genuinely creeps you out. The game becomes much less scary when there are five other people all watching your back, all shouting out the same stock phrases with the same voice actor.
In the end, I think that playing Killing Floor multiplayer might arguably be more entertaining, but going it solo was much more memorable. Now, if you'll excuse me, I think I'm going to leave the lights on tonight.
Bottom Line: Not very polished - the characters are barely any distinguishable from one another, and all use the same voice - but a solid effort from developers Tripwire Interactive nonetheless. The co-op multiplayer is intense, filled with you-watch-my-back-I'll-watch-yours moments that get your heart racing, but playing it by yourself is far more likely to creep people out. Killing Floor does what it does very competently, though it doesn't exactly knock anything out of the park.
Recommendation: Fan of co-op survival horror? If you have a spare $20, you could do a whole lot worse than Killing Floor.