I've owned this lens for about 9 months now and it is a lot of quality lens for the money. But, I find the autofocus is just a bit too slow for a lot of the sports I shoot. Also, after some dissappointing results on my 20D, I now only use this lens on my 1DmlII as it seems only the Series 1 autofocus has the tolerances to get constistant results. I was hoping to take advantage of the 1.6 crop factor on the 20D to get more telephoto out of this lens, but my 20D gives far too many soft focus results, even when my subjects were not in motion. Recently, shooting football in a poorly-lit dome, this lens on the 1DmlII did OK, but still missed on the focus all too often. I also talked with a colleague who shoots significantly more sports than I who abandonned this lens (for the big Canon 2.8 glass) after trying several copies of this lens, finding the focus just too slow.
Also, I am dissappointed with the strength of the TS-41 tripod collar, purchased for an additional $150. It really is a required accessory with this lens, even for hand-held use, as it is the only way to attach a strap to this lens. I broke the first TS-41 I bought for this lens while lifting the lens out of my camera backpack. The collar, unlike those supplied on big Canon glass, is not strong enough to rely on as a carrying handle. I'd think for the price on this collar, it could be built just a bit stronger.
My recommendation on this lens is to be cautious about its focussing limitations when considering it.