I just installed it on my laptop, and apart from having to turn the toolbars back on, it seems OK. Stuff is in different places but it makes for a more compact "screentop" so I'm not complaining.
BIG problem is that there isn't a LogMeIn version for it, so I can't connect back to the home PC from the laptop. So far, I haven't needed to - but I hope they get that sorted PDQ.
The desktop is still on 3.6.13 but it's under dustcovers and surrounded by builders at the mo. I'll decide whether or not to update it after a day or two with FF4 on this.