I've used beta versions of many software programs with few problems. But FF4 build 12 crashed so many times that I've switched over to Chrome for the time being. (Caveat: It may be that add-on conflicts are causing the crashes, but it's easier to switch to Chrome, which I use for some sites anyway, than to disable and gradually re-enable each add on to see what the problem it.) There's a new build being released this week, I think, so I'll probably give it another try.

For the few days I used it, I didn't notice it being substantially faster (on my ~2.5 year old dual core 2 laptop).