And is it fast? oh yes 
I've used Nightly Tester Tools to force compat all my themes/extensions and styles and it works as flawlessly as 3.5 and co-exists with it while I continue to check out the feature set of 3.6
I'm on Win7 x64 btw. Everything just works...also Acid 3 test
Download here: http://www.mozilla.org/projects/firefox/3.6a1/releasenotes/#downloadandinstall
Javascript speed test @ http://celtickane.com/labs/web-browser-javascript-benchmark/ gives me 200ms on the dot, 3.5 gives 297.
Page rendering seems swifter too especially when saving open media content page tabs when closing the browser and re-opening them.
Niceness!

I've used Nightly Tester Tools to force compat all my themes/extensions and styles and it works as flawlessly as 3.5 and co-exists with it while I continue to check out the feature set of 3.6

I'm on Win7 x64 btw. Everything just works...also Acid 3 test
Download here: http://www.mozilla.org/projects/firefox/3.6a1/releasenotes/#downloadandinstall
Javascript speed test @ http://celtickane.com/labs/web-browser-javascript-benchmark/ gives me 200ms on the dot, 3.5 gives 297.
Page rendering seems swifter too especially when saving open media content page tabs when closing the browser and re-opening them.
Niceness!