Wow , downloaded today, startup time has certainly improved. The address bar is just as they call it - Awesome bar !!!
New anti-phishing features intigrated, and of course, cool new back/forward button.

firefox rocks, officially best browser now, spread the news

Edit - (after 5-6 hrs of using) - Well .... the the above was initial excitement, now that I have more experiance with it, here is what I feel -

Pros -
Address Bar performs smart search on history n bookmarks using tag words as you type - looks cool but kind of bulky too but certainly best new feature.
Improved start up time.
Web site info and Anti-Phishing feature integrated, though most websites don't provide(support) much info yet but maybe in the future, anyway Anti-Phishing is cool.
Bookmark tagging system, haven't used much but looks like a nice concept. Bookmark organiser nice too.
Download manager n addons manager are supposed to be improved ( though I can't see much difference )

Cons -
I know there is a whole new rendering engine n its supposed to be so light on memory n stuff, but I felt none of that except from the faster startup. Actually I have seen a lot of instances when CPU jumps to 100% n jitters follow.
Scrolling some pages may cause excessive cpu usage n frame loss in scrolling animation (just as in FF2). Also while at pages with flash, shockwave , flv - youtube etc. I experienced too much CPU usage n jitters in video playback / flash animation etc. ( but that might be because I was on a P4 2.8g, but still ... FF2 was better )
Ctrl +/- zooming applies to all tabs from same site in window, I don't think that happenend earlier ( or is that a pro ?)
All good themes are incompatible with v3 as of now, those that are available either aren't stylish or dont implement the new cool back/forward combo button well. . But that wont be for long.

I hope most of these will be corrected with first few autoupdates, and I also installed a SmoothWheel extension for scrolling (I would like to think it is helping ).
But I like it enough that I am still writing all this in FF3 only.