This is my favorite lens for sports and also for nature photography when reach is not a big issue. It's the smallest in Canon's family of professional super-telephoto lenses. According to Canon, it features the documented fastest autofocus of any camera lens manufactured. It is rated as the sharpest optically of all Canon's L-series professional lenses. With its f2.8 maximum aperture, it will autofocus even with a 2x teleconverter on Canon's xxD series bodies, and even with stacked 1.4x and 2x teleconverters on Canon's 1-series bodies. Weighing only 6-lbs, it's only half the weight of the 400mm f2.8 lens, and is easy to hand-hold. I have done this for a half day at a time with no problem. The lens also features an autofocus preset with memory, so that you can instantly revert from the subject you are currently focused on to another one that you have pre-set. Some of the other features of this lens are listed below (from the B&H Photo website).
Click thumbnail below for sample image captured with the 300 f2.8L IS lens:   Maria Sharapova @ the Bausch and Lomb Championship
The EF 300mm f/2.8L IS USM sets new standards in performance and versatility with its lighter weight, better balance, and particularly its new Image Stabilization. In addition, the tripod collar can be removed for even more weight reduction and hand-holding comfort.
Flourite & UD elements eliminate secondary spectrum chromatic aberration
AF speed is the fastest in the world
Full-time manual focus override
AF Stop function shuts off autofocus when an obstruction appears
Dust & moisture resistance made exceptional by magnesium alloy lens barrel and new rear gasket at lens mount
Two-stage image stabilization; dual-axis camera shake correction or vertical axis only(for panning)
3-stage focus limiter
Focus preset function allows for returning to a specifically set focus by turning a ring in front of the focus ring