I've been using IE8 under Windows Vista for a while and until recently I have not seen any major compatibility issues with websites other than Sourceforge. There are warnings from Sourceforge that it is better to upgraded to newer version of IE or Firefox. But my version of IE 8 is the latest.
Today, I happened to read Linux Today and it has a link to Earthweb (part of internet.com) and IE8 is the only browser which has problem with it. Firefox 3.08 is happy. Google Chrome 22.214.171.124 is happy.