FireFox is my main browser. But I need IE 6 as a safety net, via the IE Tab extension, for certain uncooperative sites. I'm not upgrading to IE7 until I hear that all sites work in it, as with IE6.
It is definitely a memory hog: with two tabs open, it gobbled up 108MB+ of RAM. I've been using the beta install with half the RAM on another machine and opening pages was definitely faster on the other PC. I may keep this install of IE7 only because IE6 was corrupted and I was unable to delete all of my files without it locking up the application; I'm able to delete all files now (though I plan to re-install XP later this month - hopefully). I'm keeping FireFox for those sites that aren't IE7-compatible.