I just updated a customer's wordpress install from 1.5.2 to 2.5.1, including a server move, a mysql version change, and a PHP version changed, and only saw one hiccup.
I work for a an ISP/colo/high end hosting company. We recently got some new business from an old customer after a very well know domain was purchased by him -- it actually turned out to be about 180 domains that have been transfered over to our servers in the last month or so.
So anyway, one of the domains had a WP blog on it -- running 1.5.2. One of my tasks, being the company WP guru was to get this WP blog onto our server and upgraded.
After transferring the files to our server, and getting the old DB transferred I logged in as admin and deactivated all of the plugins and changed the theme over to the default one..
I then logged out, used fetch to grab a copy of 2.0.11 and went through the process of removing the old files, mv'ing the new files ..
I upgraded the db, and was done. Not a single issue.
I then used fetch again to grab a copy of 2.5.1 and repeated the process, finishing up with upgrading the database.
Again, not a single hitch, not a single error, not a single white page.
I activated the plugins, switched back to the old theme..
the media uploader had a minor issue that was resolved by editing the contactform plugin.
done, and easy as fricken pie. I dont think Ive actually said thank you to the devs for a while, and I thought now was a good time.. so thanks! :)
and to all you users, 99.99% of the upgrade problems are caused by NOT following instructions.