Haven't tried it yet, but in general i would assume the following:
Upgrading existing xp box? always a gamble - could go smoothly, could be some frustrating moments. Most drivers are still in Beta stages.
Buying a machine with it already on - much safer
Safest to wait until SP1 and vista drivers are the standard...
But i hear its nice with a few minor annoyances (the UAC as was already mentioned - but you can disable that)