I must note, my upgrading from XP to Vista was in a Virtual Machine and
surprisingly it was successful. I really have not seen or read about
upgrading experiences except from past builds, which are considered old now.
Thats part of the reasons why I decided to do it myself.
But I would assume, if it was this successful in a Virtual Machine it could
be the same doing the upgrade on Physical hardware. But again, because of
the complexities of the various installations of XP out there, it is hard to
guarantee a successful upgrade. The best you can do if you want to take a
chance is to disable any installed AV before upgrading, run the
compatibility wizard, remove any conflicting hardware and remove external
devices such as scanners, printers and download any necessary updates.