Works fine for me. As a few others above have pointed out, it def replaces IE8.
I'll play with it but I'm a pretty much die-hard FF fan now running 4.0b7pre.
IE9 is much faster in rendering motion as the FishIE, on my weak system, shows 30+ FPS at 100 fish on IE9 and only 17 or so on FF.
(Posted with IE9) No issues so far - all gadgets are working fine. Classic Shell still good to go. FF plays nice with it as I have both running right now (and fish swimming in each