I still play L4D regularly with a group of friends, finishing off the night with a hard fought win in Vs is some of the most fun I've had in gaming.
I gave Killing Floor a go last night, seems it's suffering similar issues to Demigod, connecting to a server is a nightmare. I eventually had to open tons of ports just to try and connect. It came good when a friend came along to play though as his connection is good and we were able to run a dedicated server on 1 of his CPU cores while he connected on his other CPU core and we played some 2 player for a while.
The atmosphere is superb, and when you are getting overwhelmed and attacked from behind it becomes really chilling and intense. We beat an Easy game but so far Normal is too hard for us, which is awesome cause there is still Hard and Suicidal to beat aswell. And I love the British voice overs.
I hope this server/connection issue gets sorted soon, relying on just playing with friends sucks. It doesn't even have the option "Join server when a slot becomes available" which most Steam games have enjoyed since the days of Counter Strike 1.3.