I tried Firefox 3 twice before, while it was in alpha and beta. I was not impressed at all, it seemed buggy (normal for alpha) and I found the new location bar behavior unintuitive. It did not seem that much faster either. So I always quickly went back to Firefox 2.
I tried again recently, funnily, because of a bad Kernel update. One day, after a kernel update, my laptop started to run only at a max of 800Mhz, I did not notice it immediately. I thought wow Firefox 2.0 is really slow with gmail, maybe I should try Firefox 3 again. I did and worked a few days without noticing the processor speed difference that much. Firefox 3 at 800Mhz is as fast as Firefox 2.0 at 2Ghz. It is only after struggling with very long compilation time related to my work that I thought something was wrong.
As usual with Linux troubles, solution are strange but not too hard to find out. I just had to add a processor.ignore_ppc=1 kernel parameter for the kernel to behave properly again.
Yesterday I installed Firefox 3 on a MacBook, and I was really impressed by its speed. I felt faster than Safari. And now I got used to the new location bar behavior, I would not go back (I think the behavior improved after the betas as well).
Another interesting browser these days, especially for bloggers, is Flock 2 (based on Firefox 3).