I’m using Firefox since its RC1 release. In the meanwhile RC2 was released.
The theme has changed a bit. It has more gradients now, but that’s OK for me. A nice addition is that the icon for closing tabs is now on the tabs themselves and not on the right side of the tabbed pane.
The greatest feature for me is the integrated spell checker. At least I could get rid of the Google toolbar which was slowing down Firefox considerably. Now any text I type into some textarea is automatically spell checked. From time to time I have to change from English to German and back again, but that’s easily done.
All of my extensions except the Delicious extension are working fine with Firefox 2. I really hope the Delicious extension is updated sometime soon.