Computer:
Ubuntu Linux 6.06 LTS, KDE
CrossOver Linux 6.0.0 (Trial)
Nvidia GeForce 6600, 2GB of RAM, P4 3.0 Ghz, crappy old CRT monitor
Launch Options:
-console -heapsize 512000 -dxlevel 80 -width 1280 -height 800 +cl_drawmonitors 0 +r_shadowrendertotexture 0 +r_WaterDrawReflection 0
Gameplay:
The game is playable enough to be just as much fun as it is on windows.
I've played through the game twice on windows, and this time around, the physics or something felt a bit different. The physics aren't quite the same - I could run up places I'm pretty sure I can't in the windows version, but this might just be the result of updates/patches. Overall the gameplay is roughly the same, you just might be able to get on top of stuff or jump up things you don't remember being able to in Windows. Frame rate just as good as in Windows and is smooth.
Sound:
I couldn't get the sound to work in HL2 in Gnome but it works fine in KDE. The Valve intro clip when the game starts is the only thing that really stutters. During the game you only notice a stutter every once in a while(like once every half hour).
Graphics:
In general things are rendered perfectly with correct lighting (including smoke, fire, and lights) except for 2 things: Water and Sky
Water sometimes looks alright but most of the time it flickers like crazy and is weird colors.
The sky looks fine when water is not on the screen. When water is on the screen, the sky looks like an upside down picture of what you currently see (But at least you can still see the sun :).
High detail shadows don't work for somethings and for other things they are flickery at best. Low detail shadows seem to work fine though.
The magnifying glass on the map in Water Hazard didn't work :(.
Some screens/monitors in the game don't render correctly.
Stability:
I played for a couple hours straight without problems. Right now I'm playing through Ravenholm. Windowed mode and full screen both work great.
After switching between windowed HL2 and a chat box for a while, HL2 froze. On another occasion, Ubuntu itself froze (not just gnome, couldn't Ctrl+Alt+Backspace out or anything). To avoid freezing I just don't switch between HL2 and other things often.
Overall:
The game is playable and has good frame rate, good audio, and good video (except for water/sky problem :/).