I like it
Been using it all afternoon under Windows 7 (shush, I dual boot) and it's pretty good. Can't comment on load times as the initial load was obviously converting my profile/checking add-ons, but once loaded, it's pretty sprightly. Opera and Chrome die-hards will probably crow about theirs being 0.2ms faster to load a page, but really, the difference now is slight enough that I couldn't care less. There's far more to browsing the web than rendering speed, after all. Now that FF4 is out and IE is becoming less of a hog, I don't think there's a current browser out there I'd describe as "slow".
I'm not a fan of the new minimalist interfaces in any of the current browsers, to be honest, but getting everything back where it belongs really didn't take long. (This was also the first thing I did in IE9.)
Some of the new options and fiddles are nice. Having Sync built straight-in is a boon, and the new add-ons/personas screen is very nice indeed. I also like the "app tabs" or whatever they call them - very useful indeed for getting my most used sites down to a wee pinned favicon and out of the way.
The concept of tab groups appeals, but there really really needs to be a keyboard shortcut to switch between them. I might knock together an add-on that does just that. My personal view has always been that every second spent reaching for the mouse is a second wasted. ;-)
Probably half my add-ons aren't listed as compatible, which is the real bitch. Some of these are stupid things I haven't used in ages, like Froggr (exactly what you think it is), so I've removed them, but others are useful to me, and I'm hoping that the add-on developers are still around to fix them up. That's the risk you take with a system like this, though. You can't just assume compatibility, after all. The major must-haves still work, fortunately, like Firebug and the Developer Toolbar. Thanks guys!
Incidentally, being able to install/uninstall without restart is a kick-ass addition. That always used to do my head in.
In all, I like it. There are a few tweaks that I would have made here and there, but it seems stable enough, it zips along, and it works as well as it ever did. Just got to wait for some of my add-ons to update. Looking forward to playing under the hood with the new developer tools, as soon as I get chance.
Now, over to the inevitable Opera fanboy to yell about how rubbish it is and how all the good bits were stolen from the Cult of the Chosen.