I've got Vista Ultimate 64 and Home Premium 64 running on both my machines at home.
these are both perfectly stable with no issues at at, in fact the only problem I had was when I first installed Ultimate (about a year ago) there was an issue with installing from DVD with 4GB (needed a hotfix) after initial installation with only 2GB. Home premium was installed about a month ago and the DVD was "slipstreamed" with SP1 already and I had it up and running from a fresh install to full patched in a couple of hours. I still have the original HD's with XP on incase i need them (they are sitting on the shelf collecting dust).
as for performance... i get between 40-110 fps on each OS running wow, boot speed is quicker than XP, draw your own conclusions from that (machines are Q6600, 4GB Ram, 8800ultaXT (ultimate) / GTX260 (home premium))