I downloaded FireFox 3 Beta yesterday, and I was pleasantly surprised. I don't notice any difference in the look or the icon or the bookmarks.
There are nice visual upgrades. But the number one improvement is the speed.
It runs so much faster than 2 ever did. Pages load faster, bookmarks load faster sites look cleaner, windows open faster. Everything is faster; and I haven't noticed any substantial memory leaks!
I can't wait to see the final release.