For what it's worth, I'm running the 64-bit executable of UT2004 and the game crashed to desktop within 10 minutes under Windows 8. First time I've ever experienced UT2004 crash. Using compatability mode, however, it's managed a few hours of play without problem. In addition, the horrible stutter I've not been able to solve in windows 7 appears to have gone. Hoping it stays that way!
Other than that, it feels very snappy and I'm liking the boot times. The metro interface seems unnecessary and less intuitive though. Obviously more designed for tablet use. I'd really like my start button back!