The 500mm is probably Canon's most popular b-i-g prime lens and it's one of the best telephoto lenses ever made. It's a lot more portable than the 600mm f/4, to the extent that some people claim to be able to use this hand-held. (Though we'd really be a lot happier if you use a sturdy monopod or tripod. We wouldn't like to see this beauty damaged. And nor would your bank manager!) The Image Stabilization system gives 3 stops of anti-shake compensation, and it's still very sharp with a teleconverter, so this is arguably the lens of choice for wildlife and bird photographers.
This lens can be used very well with a Canon Extender (teleconverter) and it will still deliver superb image quality. The 1.4x Extender makes it a 700mm f/5.6 lens which will autofocus on all EOS cameras. The 2x Extender makes it a 1000mm f/8 lens which will autofocus on 1D-series cameras.

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