The only test failures for IE9 AFAIK
relate to SVG files.
I personally welcome IE9. Although I have no intention of using it for anything other than testing, it's light years better than any previous version and it supports the working drafts of HTML5 and CSS3. Internet Explorer is notorious for holding back innovation on the web and with any luck IE9 will see widespread adoption among those that insist on using a M$ browser.
In fact, I timed the release of our MMO video reviews with the GA release of IE9 because it leverages CSS3 in presentation.