I've been through the Beta's from no. 2. The start was excellent, the middle a bit rough.. But RC1 is just what I was ultimately hoping for. It works for XP and Vista - even at the same time; which is handy as I have been configuring Vista to mimic XP.
It's fast and it's stable. That's all I want.
Great stuff.
(Sorry - Unity still leaves me cold. I tend to use either full screen or single window)