Doesn't look much different from IE8 so far as I can tell. Haven't used it much, but it still lags horribly behind FF3 and Opera in UI.
Only difference is that there's yet another layout engine. Fucking Microsoft. As if it wasn't enough to have to deal separately with making CSS behave in IE6, IE7, Gecko-based browsers, and Webkit-based browsers, now web developers need to make IE8 behave as well. Almost enough to make me want to stay as far away from web development as much as humanly possible.