We've recently upgraded a copy of a 100,000+ contact install running 2.2.8 to 3.0 b3 for testing purposes. Functionality is fine and things look absolutely great in the new interface.
The upgrade was painless and quite quick. There was a foreign key drop constraint in the installer that needs tweaking, but the core team is on it and apart from that it took only about 20 minutes start to finish to upgrade the fairly good sized database.
