I just installed WP3, debating using it for a website I'm working on:
http://betaphidelta.com/wordpress (drop the 'wordpress' to see the original).
The problem I'm having is that 3.0 is terribly laggy, taking between 30 seconds and 2 minutes to load a page! Admin console is no better.
I have another wordpress blog http://games.ajcvisual.com and haven't upgraded this to 3.0 yet, it doesn't have any real lag issues.
Both sites are hosted by ipower.com, but are on separate ipower accounts. I've thought about simplifying my DB, or compressing the code, but am not sure if this is even necessary, especially with one blog working so well. My other solution is to downgrade.
Any idea what the problem is? Check the sites for yourself!