I'm using it, It's alright, nothing special *shrug* a few hanging issues at the moment, but I;m sure that will be ironed out by final. I have always used Firefox, and I won't change out, but apart from Direct2D and HTML5 it's not a massive leap forward. I don't like the UI either.
@maranello, it will install along side, but will use your FF profile, so your awesomebar etc. will remember everything you did on the 3.* version