I just upgraded to WordPress 3.0 "Thelonious" and I have to say it is a great upgrade. I haven't had any problems and the default theme Twenty Ten is just wonderful. If you are just upgrading your install, you will need to use the theme uploader to install twenty ten, but it is worth it. It puts many of the themes out there to shame. It is what I've been looking for.
If you are normally slow to upgrade, don't worry. Things tend to be working great.