i myself have a big video project (for a contest) and since my last clean reinstall i had to work too hard to get even the correct codec for the videos, and talking about the videos, i had to define their new location manually... so i'm going to try out the upgrade, but i will still try to atleast prepare for the clean install, because nobody is perfect... not even microsoft. and what i've learned when i tried windows vista developer preview: Risking important files is stupid, when you can just pack all the files into one 7z / zip file and remove it afterwards if everything went fine.
But i'm going to definitely go and try the upgrade (just the way i explained above)