I've been using the release candidates for awhile, and all I can say is, it's been a vast improvement over Firefox 2.0+. The new version is much faster and uses far less memory. The "awesome bar" (granted, it's a terrible name) takes a little getting used to -- although you can always use it just like your standard address bar -- but has definitely sped up my browsing. Firefox 3.0 actually learns what you're searching for, based upon your browsing habits.
The stars and tags for bookmarking is also quite handy, especially for those of us with out of control bookmarks! Not to mention lots of new security features, including phishing and malicious website protection, and my favorite: a single-clicks on the favicon part of the awesome bar gets detailed security information about the site you're currently visiting.
Oh, and since I am an extension fiend, I'm very happy that the vast majority of my extensions are already updated to work with the latest version of Firefox.
Enjoy, and stay foxy!