Ran Steam OS for a bit on a franken-computer in the living room.. q6600, gtx460, 8GB of 800MHz RAM that were all lying around collecting dust. It worked fine, but the drivers weren't the best (I imagine they'll eventually fix this.). Didn't get very good frame rates in most games. Out of my 250-ish steam games, a respectable 78 were linux playable.
Ended up just modding a very basic windows install to boot straight to Big Picture. All my games run fine now and I even have the Steam Music beta.
If you want SteamOS, just use a minimalistic Windows install in BP Mode.