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.
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.