I use XP for testing purposes... and I runs very well... except the "emulating modes" for games (like Colin McRae racing) and stuff. If I want to try that, the games aren't playable that's why I use a dualboot with 98SE, and also to burn most of my CDs, 98 comes in handy... there are a few major bugsin XP, that you will never experience if you have a big C-partition where XP is installed:
the problems are:
-try to move (or resize) the your swap-space (=pagefile.sys) from the C-partition to another one... well... you can't really!! you change the setup and nothing changes!!
-try to move the place where your temp files are being placed to another partition (not on the XP-partition)... well the soft says: you can... but look at your (free) space of your XP-partition if you are getting your files ready to burn on the CD... your space will diminish...
-1 other stupid bug for some motherboards: the ATX powerdown doesn't work > ...
other bugs... I didn't find any