The challenge that Microsoft will need to overcome is how compatible IE 9 will be with websites. When IE 8 came out it wasn’t compatible with many different website.
If you would like to try IE 9 you will need to be running either Windows Vista and 7, XP is not supported. Lastly, you were able to run the beta of IE 8 side by side with IE 7, but you can’t run IE 9 side by side with IE 8.
Read more at CNET