Firefox 8 definitely feels faster than Firefox 7. I'm using it since today in Linux (it's now in Portage), and since a couple of days ago on one of my machines running Vista. I can't see any other Earth-shattering changes so far, but Firefox 8 is working fine in both OSs.
By the way, the AVG Free anti-virus checker on my Vista machine occasionally used to pop-up a warning message that the memory use of Firefox 7 had crept up to a threshold value, and recommended that I should restart Firefox 7 to reclaim some RAM. It'll be interesting to see if I still get those messages now that I'm using Firefox 8 on that machine.
EDIT: For KDE users of Firefox, a new version of the Oxygen KDE
add-on has been released for Firefox 8. It makes Firefox look even more like a native KDE application than the KDE oxygen-gtk theme.