I use Firefox almost exclusively, and anytime I use IE is in the context of the Firefox plugin "IE Tab." I've also made sure the whole family is using Firefox as well, mainly because the number of nefarious exploits out there for Firefox are significantly less than for IE (that is slowly changing as FF gains popularity). Well I just installed FF 2.0 and so far it's been working great. The UI updates are very subtle, but I can tell the overall performance is definately better. Memory footprint is about the same, though.
I happened across this posting where the Internet Explorer team sent a cake to the Firefox team congratulating them on the release of FF 2.0. Of course Slashdot is filled with a myriad of conspiracy theories, negative spin, and outright paranoia... however my initial thought was how classy that was for the IE team to do that. The FF folks should be proud of their accomplishment and the fact that a competitive team from MS is recognizing the tireless efforts of the team working on an open source project, speaks volumes... about both teams.