It's actually really good. Unexpectedly, even. I figured that they would have ruined it by now (2.0 betas were impressive, as well, but the closer they got to releasing it, the more and more it slowed down) but they really haven't. Compatibility with already existing extensions is pretty strong. I've been told that the changes on OS X are a lot more relevant than the changes on PC (since they finally unified Firefox with the rest of the OS environment, etc.) but pages still load generally faster on PC.
Something about using Firefox still feels wrong on the Mac, but I'm sure that's not a problem on PC.