I did have an error similar to that for the shortest period of time possible (HP Pavilion m6 1009tx with Radeon 7670, but I cant remember for the life of me what it was about, because I rebooted after some updates and it was gone. Steam is still clunky for me on a couple of Ubuntu-derivatives (it sometimes exists in the sytem try but when minimised it wont disappear from the tool bar), but is definately functional. I'd love a WINE module to tie in to it for the Windows games that I have that wont come across. I'm pretty sure running the windows client would fix this, but eh Later when the 3.12 kernel I am running settles down and I have my mic back.
A couple of annoying bugs I have are generally related to the games themselves (ie DOTA2 NEVER seems to show prizes no matter what linux-based system I'm running), audio seems to stutter a bit after selecting a toon as the game hangs a little bit (possibly network latency related or likely kernel as seems to be only recent) and performance doesn't seem to be as good (however, I am using x-edgers FGLRX and a 5400RPM drive so there's a definate load time-related thing there compared to my nVidia/SSD desktop)
And Big Picture looks like a kiddy built it. It's got nothing on the slickness of XBMC's visuals.
However, all in all, Steam has shaped up to nearly as good as the Winblows client, and in far less time comparitvely.
If L4D2 is the only game giving you trouble then it's not your graphics card that's to blame. An AMD 5750 should be able to handle L4D2 with no problems.
What kernel are you running, and your mesa version? (I'm not familiar with your version of openSUSE - "$ glxinfo | grep OpenGL" will give the latter) What graphics options are you using in L4D2?
LFD2 runs better on windows, under DirectX. To name another game Serious Sam 3, no problems at all. ( it might even run better on Linux, but without benchmarking, I can't tell for sure.... )
And both games have at least one thing in common, the spawning of shit-loads of enemies.
SS3 works greats since the amd beta driver which supports openGl 4.
Before this the game was almost un playable, if I did not turned down the resolution allot.
Teamfortress 2 runs perfect too. ( so we can also rule out my internet connection 50MB down 2.5MB up )
The halflife benchmark, lost coast runs FPS close to windows, although its a tiny bit more fluent on windows, its more then good enough on Linux to have fun.
LFD2 has very annoying short freezes, or should I say lag, mostly noticeable when the zombies attack.
Further more I like to add, that the amd drivers have improved allot over the last year.
I am running latest catalyst beta drivers.
glxinfo | grep OpenGL does not seem to do anything.