When you know you're going to be working at 200mm, this prime lens makes a very attractive alternative to the various 70-200mm "L" zooms. It's smaller, lighter and much more discrete than those big white things, and it is very sharp as you'd expect from a prime.

This lens can be used very well with a Canon Extender (teleconverter). The 1.4x Extender makes it a 280mm f/4 lens which is much smaller and lighter than the 300mm f/4 L IS. The 2x Extender makes it a 400mm f/5.6 lens which is smaller and lighter than Canon's 400mm f/5.6 L prime. With either Extender, the lens will still autofocus on all EOS cameras.