Killing Floor is a pretty sweet game. I don't understand why it gets so much hate all the time from communities. The weapons are all cool, the zombie types are generic but well managed, the maps are interesting, the graphics are great considering it's running on UT2004's engine. It has plenty of custom maps by now and community support.
Also, L4D and Killing Floor are two different style of games. Left 4 Dead is focused on protecting yourself as you move from checkpoint to checkpoint, whereas killing floor is wave-based combat with a item purchasing and upgrade system.
Killing Floor is the kind of game that everyone who has played it, loves, and everyone who hasn't just thinks it's some me-too budget title with poor visuals. It really is well worth the money if you're into any sort of multiplayer gaming, and even I'm not really, but I still had tons of fun with it. It has literally hundreds of maps available, most of them extremely cool and fun, and there's a pretty active modding community to boot. Far more replayable than the likes of Left 4 Dead, certainly, and I find it more strategic and fun on a moment-to-moment basis; it's less about luck and running around like a headless chicken, and more about true teamwork and cooperation, without making individual skill worthless either.
Expect a huge influx of noobs into this game, considering it gives a TF2 hat now. I bought it during the summer steam sale and played it a few times. Enjoyed it quite a bit, not sure why I stopped playing it.