Hi.
I use IE7. Wokrs very fast, but as you might expect, I took a lot of time making sure it was set up the best way to match my usage.
I won't touch Chrome as it is very bugy and has major security flaws, judging from some of the reviews I've received. Also it uses a HUGE amount of memory when running. More than XP itself, apparently.
FF is OK, but I'm not planning to swap from IE. Nothing in FF tempts me to do that. I keep it for compatability checks on my sites.