The 300mm f/2.8 L IS USM is one of Canon's very best lenses. Optically it's about as close to perfect as you'll find (Except for the newer and more expensive Mk II version which is even closer to perfection!) It's not too heavy, and the excellent Image Stabilization system delivers 3 stops of anti-shake compensation, so it's very popular with sports photographers. It can be used hand-held, but a tripod or monopod is recommended.
This lens works very well with Canon Extenders (teleconverters) and it will still deliver superb image quality. The 1.4x Extender makes it a 420mm f/4 lens and the 2x Extender makes it a 600mm f/5.6 lens; both of these combinations will autofocus on all EOS cameras.