Basically you can play the game with fglrx as well but you might need the lowest gfx settings to get a high framerate. With faster cards you can enable a bit more, but when the speed drops you get very extreme input lag. That does not even happen with slow nvidia cards, even with low fps. But it was a really cool gift, i hope much more Linux users can play the game now. The engine is older than Team Fortress 2, so it is a bit different.
Runs perfectly well on an A8-6500 with the OSS drivers (i have compiled mesa, drm, xf86-ati and kernel from git). I just played a bit (until chapter 3) and it ran perfectly smooth with vsync on at 1440x900 even when there were tons of zombies around. No input lag.
The Source games i played so far (TF2, DoD Source, L4D2) all ran perfectly well on the OSS radeon drivers. In fact the Half Life 1 based games (at least Day of Defeat 1) run slower for some reason.
Hey, if my keyboard could handle more than two keys at a time I could easily play L4D2 with the open driver and mousekeys. ;p
Seriously, though, runs great with settings maxed (except AA) and 1080p. Only thing I noticed was the mouse continued a little past when I stopped, but I think that may have more to do with my sensitivity settings (I haven't tweaked those much yet) and the fact that I'm using a bluetooth mouse.