I have also noticed the font problem, and there is no real fix as of yet. Hopefully soon..
But yes, it is much faster, even on already fast machines!
You can switch the tabs back onto the bottom, so getting used to the UI is easy.
A great tweak, since BarTab doesn't work right in FF4, is to type this into about
:config
browser.sessionstore.max_concurrent_tabs
set to 0.
This will make it so your stored tabs won't load till you click on them, so if you're like me and keep 30 tabs open, when you load up firefox, it won't take forever

also if you don't use them for a while it will take them off your RAM.
Still looking for a nice black theme, since the NASA night launch is incompatible.
So far I like. Granted that they fix the font problem, this is a sweet browser that is back in the times with chrome.