I'm running IE7 Beta 2 on Genuine XP Home.
It's not too bad actually, just a little bit slow and buggy, but MS'll get there eventually. Sometimes it just packs up and stops browsing to pages altogether, for example. FF still works, so it's not my connection (happened on several different connections too).
However I don't like the new interface. Far too confusing. Not sure I like the new fonts either though, everything just looks a little too clean and shiny, makes me feel a little bit stupid TBH.
But at least it feels different - IE 4,5,6 just looked (mostly) and felt exactly the same. I see by calling it "Windows" Internet Explorer they're going the same way with "Windows" Live Messenger, instead of calling Messenger 8 "MSN Messenger 8", where they could have called IE7 "Microsoft Internet Explorer", as they always did.
I'd love to try it on Vista, shame I don't have it :-( I know a guy at Uni who's got it, but I don't like the guy much so I don't want to go to his room and try it.