I've upgraded 2 machines, a Lenovo Laptop and a "homebrew desktop from 7.10 to 8.04 - and did it the risky way, with the "Update Manager".
I was too lazy to burn a CD !!!
Both have been running for a couple of weeks with no problem (knock on wood).
Actually, the upgrade "fixed" a video glitch I had on boot-up ...
Sometimes you are the bug and sometimes the windshield !!!