It's a seriously fun game. And well worth the $20 I think. There are already a ton of custom maps (not to mention all the maps being made for the mapping contest Tripwire is running for the game) and even modded servers that allow 30 or so players instead of the normal 6.
I loved the mod version, and the retail version is SO much better in my opinion. It could use a bit more polish here and there obviously, but overall it's a really fun game. I'd actually much rather play Killing Floor than Left 4 Dead.
Also, like with Red Orchestra, we're gonna get quite a few updates for the game I'm sure. We've already got a paid DLC out (skins/models for a paltry $1.99) and shortly they are going to release free DLC with a new map and two new weapons (AK-47 and the Katana). Can't beat that really.
God I sound like a salesman...