I've just installed FireFox 2.0a2 Bon Echo and I'm mightily impressed. Truth be told I've never got on well with 1.5. It was always extremely sluggish for me and a bit of a resource hog, both in terms of memory usage and CPU cycles. I'd even gone as far as to semi-migrate to Opera 9, which while incredibly fast and efficient, I missed my favourite extensions (Web Developer, CSE HTML Validator, greasemonkey, Gmail Manager, AdBlock).
I could live without some and there were workaround replacements for others but now I can see what is up and coming with FireFox 2 I'm willing to stay put. Writing this post I notice it now has a built in spell checker, which behaves very similar to MS Word's and is worth the upgrade alone! The UI has taken more than a few pointers from Opera (close buttons on tabs, search box) which is certainly no bad thing, although I'm glad they haven't changed the order in which tabs are focused on once one is closed. That drove me mad in Opera!
I've installed the Nightly Tester Tools to get all the 'incompatible' extensions working and only RoboForm has complained which I was considering ditching anyway. I'd recommend giving it a try, especially if like me you found 1.5 really disappointing. Here's a summary of the big changes.