I had a few problems with it, but that was due to plugins more than anything else. WordPress 2.0 is finally out, and it is working rather well for most people.
There is some great information on the WordPress Support forum on the upgrade and whatnot.
It is very important to read this thread as it contains information that might save you many headaches.
Backing up your database is ESSENTIAL.
Read the instructions and get the information at WordPress.org/support.