I just installed it yesterday. Very fast, and the leaner memory hit is amazing!
Can't say as much about the new default theme, though. I am not that much into skinning - why use up this newly freed memory with graphics? But the new look is pretty bland. And there are not that many add-on skins available for it yet. Be a while for that, I guess.
Also, about half my extensions worked right off the bat. I do have the Nightly Tool Tester extension installed, which can "force" a compatibility update for non-working extensions. But it will only work, of course, if the core extension CAN work in the new version. If not, the browser can get pretty broken. So I am trying to determine that others have successfully gotten the extension to work elsewhere before I just dive in and do it myself. So far about a quarter of my favored extensions are still not working.