I am running a dual-boot XP/Vista system. The Vista was installed in a clean partition: it started with Beta2, then upgraded to RC1, finally the retail Ultimate system which was a forced new install; the upgrade from RC1 to RTM was not possible.
I have had very few problems with either one of the versions, just a few driver problems; even now I do not have a complete HP-printer driver; it works somewhat...but that is the only one. Driveragent.com has been helpful finding the correct drivers!
Finally, someone at MS told me that with Vista you can activate one key max. 10 times .... after that you have to buy again.