I decided I'd upgrade to 5 because I like some of the new things. Overall, I'd say it's pretty nice.
I had to turn off hardware acceleration to get the font to not hurt my eyes, even if I could tune it with an add-on to make it look considerably less bad, the spacing was still weird and it still burned a little bit for some reason.
My favorite theme isn't available for FF5, of course... So, I had to just make something close to it with a combination of Personas+Colorful Tabs. I made the background the same then set the colorful tabs to a single color, with a bit of fading. I wish I could set the color of the active tab, that'd get me a lot closer to my old theme. But, it's alright I guess.
I really like the tab grouping and app tabs.
I, too, made it look like FF3 (I also
disabled WebGL just to be safe). I never could stand the way Chrome looked. It pissed me off that Firefox copied them.