Amazing! Nothing like playing Left 4 Dead 2 on simply the best gaming platform (dat performance). TF2 ain't too shabby either, and neither are the billions of mods for the Source engine.
It's like the worst-kept secret ever, but as soon as Desura began booming on Linux, I guess Valve didn't have much of a choice other than porting their games to the platform of the future.
Gotta say I'm kinda disappointed it's Ubuntu-only, which may not be compatible with Debian. Still, Ubuntu-based distributions are some of the best desktop distributions around, no question about that. And there are lots of support materials available for them too (Ubuntu forum just to name a huge one). This is why the Linux Standard Base is so important (for god's sake though make DEB the default already, it's just used so much more). Particularly that aspect.
Also kind a annoyed about the last paragraph because whilst Valve releases quite a few of their games and tools as freeware, I'm almost certain they've never released anything under an open source license. I'll let that slip just this once.