well will see. I have been testing IE 9 since it's RC version and I have noticed there were lot of changes so far. infact IE 9 supports JavaScript 1.8, HTML 5, but still no Canvas Tool that means you will never be able to render OpenGL animations based on Canvas tool+javascript. It's actually supports with FF 4,Chrome 10 ,Opera 10,11 I think.
IE 9 got lot of security improvements .one thing is you can now select the security functions on top of the menu bar. it has dubbing tools that may usful for developers.
I didn't see anything wrong with IE beside Canvas tool. it's really faster than even FF. and has also Microsoft's clear type technology which improve the quality of the contents.