FF3 has a few new features that I like. One is the ability to back up your Bookarmarks easily. Another is to zoom in on the page for the older-than-dirt eyes of mine.
I am on a Mac, and have had it crash about every 4th day since I installed it the same day it came out. I expect a point release within the next month.
So far so good.
I haven't tried Opera in years. I'm happy with Firefox, so I doubt I'll even download it to try it, unless djy can come up with some reasons why I would want to.