I use Opera but have to use IE for facebook apps as some of them don't work properly with Opera.
Recently I upgraded IE6 to IE7, then IE8. Both IE7 & IE8 are totally useless so I downgraded back to IE6.
Both IE7 & IE8 have the annoying habit of forcing the window to the front when the page loads. Facebook is so slow that I always have other stuff open in Opera, and facebook in IE. I'll click a link in facebook then bring Opera up and continue doing stuff there, going back to facebook in a while when the page/app has loaded. With IE7/8 this was impossible - I'd click the link in facebook then go to Opera and immediately IE would force itself to the front, then it'd freeze for several seconds because facebook uses too much flash or java or something and uses 100% cpu (the freezing isn't nearly as bad with IE6). IE7/8 kept forcing itself to the front when I was trying to do other stuff in Opera, it was making a mockery out of multitasking so I downgraded back to IE6.
In short, if you have IE6 don't upgrade because IE7&8 is shit.