I've had a go of Killing Floor through the Linux Steam beta client, but performance is pretty poor.
Kubuntu 12.10 64-bit (dual-boot with Win 7)
AMD Catalyst 12.11 beta x64 linux drivers (fglrx)
AMD Phenom II X4 965 @3.4 GHz
XFX HD5850 1GB
8GB DDR3 (1333MHz)
I've just been playing solo. It says I'm getting around 25-40fps, but it stutters a lot and as such feels a lot less. Turning the settings down makes a difference but it's very minimal. As such it's barely playable. When I play in Win 7 it never drops below 75 fps with v-sync enabled with all setting maxed out. This game runs on a 7-year old engine and the graphics are fairly dated, so I dread to see what performance will be like on more taxing games.
My AMD gfx card has served me well these past two years and with my current set-up I've been running current games in Win 7 smoothly at close to max settings (eg Hitman Absolution, Far Cry 3), but as a Linux enthusiast I will almost certainly be buying a Nvidia card for my next upgrade if AMD's linux support does not drastically improve.