I've been using IE (again) since version 8, because I was fed up with Firefox occupying about 80% of my RAM, especially on my netbook, starting slowly and crashing. It didn't crash often, but more often than IE, and when it crashed, it took all the tabs with it.
Also, IE uses a Win7 security feature no other browser uses (as far as I know). When IE8+ is running on a Win7 PC, it runs with lower permissions than a standard user, not quite sandboxed, but with highly restricted read/write permissions. AdBlock is integrated as well (called "Tracking Protection". Just have it use the right block lists and your web pages are clean.
IE starts a lot quicker than Firefox and offers jumplist functions (but maybe these two problems got fixed in FF4).
And: Silverlight is awesome, and doesn't feel as fluid in FF. Let alone Safari (which I - embarassingly - actually used until I realised that "Apple" folders kept popping up all across both my harddisks)