WordPress MU was just recently released and I've upgraded several of my websites without any problems or error message through out the upgrade process. It even includes a nice horizontal toolbar on top in addition to the sidebar. I'm very happy with this release for the same reasons mentioned in my previous post regarding WordPress 2.7.
The only real bug I've found is with upgrading plugins. After upgrading one plugin, the upgrade options disappear for a day or so ( I haven't actually timed it). I'll still see a number by Plugins in the menu, but the options on the plugin page are gone! This bug first appeared in MU 2.7 "bleeding-edge" when regular 2.7 came out. I reported it informally at the time.
Another thing that is needed for WordPress MU is a way to select all users and all blogs on the page without clicking each one. This is important for not wasting time deleting splogger registrations. I've been fighting the battle and losing against sploggers due to the lack of effective solutions. A simple CAPTCHA is not enough and after several months of this drudgery I've removed the signup link and I've disabled user blog signups. Hopefully in future versions we'll be able to limit signups/per day/month by IP and other tools that could help. Currently in WordPress MU you only have the option to list domains you want to blog signups for, but I've noticed that since .info domains are cheap for the first year, a splogger could easily register hundreds of those. As soon as I block one, they register with another.