FWIW, I've been running XBMC for a while, and other than some UI stuff I don't like, and some lockups that are almost certainly to do with the fact that I allocated far too little space to its root partition, it's been a champ. I don't think that it's yet failed to play anything.

I also have it PXE booting and mounting its drives as iSCSI, so the only moving part in the system is a small case fan.