I've had this lens for about a year and use in a lot for shooting indoor sports. The aperture is very handy when I'm shooing in dark conditions, and produces stunning results for portraiture with excellent bokeh.
For all intents and purposes this may as well have a red L band. It really is incredibly sharp, and probably better than my 70-200/2.8L. In fact, an example shot from the Canon 1Ds mkII on Canon.co.jp used this lens - and that's to show off Canon's flagship camera.
As well as my own review, I've heard a number of other professional photographers using this lens for indoor sports. It really does kick ass.
I was actually considering buying a 85/1.2L instead of this, but having checked out the reviews on this site, the lens seems much too slow and soft for sports. I'm not a bit disappointed with this vs. the 1.2L.