I updated on one computer from windows 7 ultimate 32 bit- took me about 2 hours, had to install new drivers
and new updated applications, then Microsoft issued the last set of horrible security updates and Windows 8 crashed. I had to revert back and start all over again- now that was a pain.
I also upgraded from a laptop running windows 7 home premium, now that did not go too bad- go figure and it was older than the desktop. Took about an hour
Do not upgrade if you are not a advanced user
Make sure if you do upgrade, you use the compatability tool that comes within the upgrade and listen and fix the issues before you do the upgrade.
My personal opinion:
If you are in love with "Apps" and like the new look then go for it
If you the least bit skeptical wait till there is service pack because there are lots of bugs in it that need fixing.