During my time using a legit copy of Vista, I had activation issues three times, resulting in Windows refusing to accept my product key until I called Microsoft to talk to a technician.
What did I do to get this sort of behavior? Absolutely nothing. The problems started from the very first initial install activation, with it saying it was activated and not-activated at the same time. It only got more and more bizarre from there.
While I'm sure they've worked out a lot of the quirks, it's still a royal pain in the ass to deal with and I don't like the idea of my OS calling home frequently either. Especially with no way to tell what info it sends.
I'm seriously thinking I'm going to keep my gaming rig on XP and put a desktop machine permanently on Linux next time I upgrade. The news about Win7 hasn't been encouraging.
That's MY experience on the matter, and I think it's pretty well in line with what a lot of other people have experienced.