Well, I just decided to try FF3 for the first time today (after backing up everything). So far, a few of my must-have add-ons are not working. I found a dev version of one that seems okay. One addon that claims 3.x compatibility doesn't actually work, so I'm trying an alternative that sort of works, but is much slower than the other addon in FF2.
FF3 locked up on me while typing a search into Google the first time I opened the browser. Not a good start. I had to kill the process. Since then it's been working okay.
I went through and deleted and disabled all the new features that clutter up my normal layout.
So far, I haven't noticed any improved performance. The only changes I've noticed are that some of the old FF2 features are slower and there is more clutter and consequently less information on screen (more scrolling and button pushing to see info that was available on one screen in FF2).
I'll give it a week and see if there are any catastrophic deal killers. So far, it looks like I'll be able to live with it. I'll have to upgrade anyway when they EOL FF2, so I might as well get used to it now.
I haven't seen any new features, performance or layout changes that would lead me to upgrade if I didn't have to.