RC1 is out now. I've just installed it, and so far so good. It disabled my incompatible addons, but I got them going again easily enough with the Nightly Tester Tools addon.

Not a whole lot has changed at first glance. The UI is slightly different, and it's a little more responsive. No complaints!

Release notes and download can be found here.

there's also a delicious add on!! yayayay