This lens is probably the favourite amongst professional motorsport photographers in the UK. It has an interesting focal length range, coupled with the constant f/2.8 aperture, which means there's nothing comparable in Canon's range.  Build quality is superb, focussing is fast and accurate, and the lens is very sharp. If Canon built this, they'd paint it white and call it an "L" - it's that good.

This lens works very well with Canon Extenders (teleconverters) and it will still deliver superb image quality.  The 1.4x Extender makes it a 170-420mm f/4 lens and the 2x Extender makes it a 240-600mm 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.