As John mentioned, the availability of specific add-ons can be a gating factor. It certainly was for me during beta. I did not start using FF3 regularly before RC1, and I had to get some alpha level FF3 plug-ins at that.
Since I have moved to FF3 I would not want to go back.
The improved rendering in the new Gecko plus the truly awesome awesome-bar improvements are features I do not want to live without. Typing keywords into the address bar to find previously browsed items is something I have more than grown used to. I just do it automatically now.
e.g. What was that article I was looking at on "Sarah Vaughan"? No problem, just type her name into the address bar and the items from history are shown immediately.
e.g. I need to zoom this site to read it easily. FF3 remembers my last zoom for the site and applies it automatically. Makes it so much easier while I am checking my RSS feeds via Google Reader.
Some people have complained that the suggestions are in-appropriate. It can happen, but, simple to fix. Just shift-delete the unwanted suggestions and they will no longer appear.
Even with the potential for problems with specific extensions, the improvements noted, plus, the speed and memory utilization improvements make FF3 a no-brain update (for me anyway).