It's white, it's fat - it could be the owner of LensesForHire, but in fact it's the ultimate sports lens.  Optically, the 400mm f/2.8 IS is about as close to perfect as you'll find.  It focuses in a split-second and it delivers razor sharp images.  What more could you want?  Some time in the gym, perhaps, if you're thinking of using it hand-held.  This lens is heavy.  Most normal people will want to use it with a monopod or tripod, and a heavy-duty one at that.

This lens works very well with Canon Extenders (teleconverters) and it will still deliver superb image quality. The 1.4x Extender makes it a 560mm f/4 lens and the 2x Extender makes it an 800mm f/5.6 lens; both of these combinations will autofocus on all EOS cameras.

Suggested extra: You might find the Manfrotto MN393 long lens bracket useful when mounting this lens on a monopod or tripod.