I finally made the upgrade to Firefox 3. The speed enhancements are incredible - it screams! Other notable improvements include an incredibly improved address bar (it's very Apple-Spotlight-inspired), a very nice new download manager, and some significant usability enhancements to the add-on manager. MozillaLinks has a thorough review of the changes.
Unfortunately, like all Firefox upgrades, there are quite a few of my favorite add-ons that are no longer compatible. Hopefully they will be updated soon, because now that I've used FF3 for a few days, there's no going back now!