[dba-Tech] XP -> Windows 8 Upgrade

A friend of mine is about to upgrade a number of his client's computers
from XP to Windows 8.

Has anyone done this? Are there any known problems or hassles that he might
face in doing this? I haven't done this, so I didn't have a ready answer.
My experience involves Windows 7 -> 8, and that was about as painless as
could be.

Any advice I can pass on to my friend would be greatly appreciated.

-- 
Arthur