OK, don't laugh: I bought this for sports after I realized I couldn't stand the stiffness and reverse direction of the zoom ring on my Bigma on my old XT. The Bigma (Sigma 50-500) was just not usable without breaking my hands. This was the light alternative!
The usability is great; I love the push pull design. Problem is, I've moved on up to a 5D mk II, and suddenly my pixel peeping 100% crops ain't so hot, even in the center! (I'm used to using my old 70-200F4 IS, which was unbelievably sharp on my old XT or even my 5D II). I mean, I can live with a little corner blur at the long end, but I need player's faces in the center to have some detail.
I've also figured out that the 5D mk II is not very useful for sports. I'm gonna get a used 1D mk II to pair with this beast, and have the 100-400 serviced by Canon. I hope they can salvage it; I'd rather own one great Canon telezoom than two.
The 70-200 F4 is worlds beyond this lense except for the obvious issue of reach.