WordPress 3.0 is out and available for download. You can check it out here.
I have installed it from scratch for one project and I have installed the upgrade using the built-in upgrade tools. Both have been very successful.
The new interface improvements are great and it seems very stable. All of my existing templates and plug-ins worked without a a hitch. The only issue I had was with the widgets associated with my templates. I just had to re-add them.
Great work from Matt and the team at WordPress.com and Automattic!