After only a few short hours online with the new beta release of firefox 4, I can already tell I'm going to like it. I have yet to find any issues, and it loads pages lighting fast. I highly recommend anyone thinking of testing the waters with this preview release to go for it.
I did have to install the compatibility reporter addon to get things like firebug, yslow, and operator to work. But I think it is fair to expect addon developers to need some time to catch up when running a beta release.