Just used the beta tester plugin to upgrade a test install to the latest nightly. My first impression was, "ahhhh that's better."
When I heard there was going to be a major upgrade to 3.0 release, I panicked slightly, as I have had problems with upgrading before, so I thought it's about time I gave 3.0 a go.
The lighter admin area is just such an improvement! Made me not so worried about the upgrade, even though a colour has nothing to do with any potential problems.
The merge with MU is a big step, and it took a little time to get my head around. It would be nice to be able to manage multiple root domain sites, or at least have a link to switch between the two. Regardless, I like wordpress 3.0, and look forward to the full release.