I've been using Firefox 3 for a few days now and so far it's worked very well. To be honest, I never knew why folks thought FF2 on the Mac was ugly. I didn't get it. But now that I've been running FF3, I see that it is a bit nicer in form fields. That by itself is not enough reason to get FF3, at least not in my opinion, but other aspects are nice as well.
Here is a great change: In FF2, you enter a username and password, and FF prompts you with an ugly alert asking you if you want to save it. While saving authentication info is real handy - the alert can be a real pain in the rear. In FF3, this is done with a prompt on top of the web page. It doesn't stop your form submission.
In general - FF3 also just seems to behave better. My FF2 had improved quite a bit since disabling Firebug, but FF3 seems to have made it near perfect.
Speaking of Firebug - most extensions I had did not work in FF3, including Firebug. I found a modified version here though:
I've been running it for 10 minutes and so far, so good, but I don't think it is an "official" update. The only other extension I really miss is the Web Developer toolbar.