I just installed it a couple of days ago (after the last version of Firebug refused to install itself on 3.6), and so far I really like it! They overhauled the UI, it got much better while still somehow feeling Firefox-ey.
This is one of these rare times where a newer version of software is actually faster, lighter, more responsive, simpler and more elegant then the previous version! Don't you love when that happens?.
It is more stable and responsive then 3.6, I guess in great part because Flash now runs in it's own isolated process, and cannot take the browser down with it. I also applied the "dark fox" style, which I thing looks great - with my dark XP skin, hopefully I'll post a screenshot later. You can apply styles without restarting the browser.
</nerd joy>
I updated today, its awesome.
The update was quick and easy and all my extensions are working fine.
just updated 2 minutes ago. all seem good
Although I didn't really like the Beta, I think I should give the final release a try following the feedback. In fact I wasn't even willing to even give it a try, as I'm happy with Firefox 3.6 and Google's Chrome 10.

On the other note, I don't want to hijack the thread, but is it me or does Chrome 10 crashes a lot with flash content? There seems some issues with the shockwave plug-in.
Okay, I left Firefox open overnight with 10-15 tabs, and when I come back, huge mega big memory leak. Firefox seems to be frozen, the system's resources drained... I kill it instead of waiting much longer. Chrome is going to be my primary browser after all, it plays much nicer. I'll still be using Firefox for Firebug and other nice things about it.
Rolf, you don't need firebug. Open Chrome, then CTRL+Shift+I. Enjoy =).
I updated today to firefox 4 to be able to use google body browser.

Didnt have any problems with the beta and the body browser is an amazing feature.