I used Fx4 for a while, love panorama and the new interface is nice, but I was getting 20+ second cold startups and ages-long pageloads prompted me to do a side-by-side of Fx and Chrome on my rig. So now I'm using Chrome as it starts up and loads pages much faster.
I like the individual-process-tabs thing, so that when one tab locks up it doesn't tank all of my others (it's convenient). It also really doesn't use too much memory compared to Firefox.
Then again, everyone's computer is different.
IE is still shit though.
IE9 has the CSS capabilities of Firefox 3.6, and all of the security of a sliding screen door.