As of mid-2013, this is effectively Canon's new flagship lens. The lengthy delay between its original announcement in 2011 and its eventual launch seems only to have whetted appetites for what should be one of the best wildlife and sports lenses available anywhere.
Although rumours of a replacement for the very popular Canon 100-400mm L zoom have been circulating for years, this lens is not it. It's much, much better - and so it should be, given that at launch it costs 10x what the 100-400 does!
Tests show that this lens holds its own optically against the very best zooms and the very best primes. Image quality is simply superb, as is the build quality, autofocus speed, image stabilization, etc. etc. The only real downsides are the weight (it's a bit heavier than the 500mm f/4 Mk II, so it really needs a monopod or tripod) and, of course, the price.
The lens has a built-in 1.4x Extender which can be activated by simply throwing a switch. The Extender makes it a 280-560mm f/5.6 zoom. In addition it is physically compatible with existing (not built-in) Canon Extenders. A 1.4x Extender gives you a 400-800mm f/8 lens which will autofocus with a 1D-series camera or a 5D Mk III; a 2x Extender gives you an amazing 560-1120mm f/11 lens, but that combination will be manual focus only, regardless of what fancy camera you have.
NOTE - Switching the internal Extender in or out whilst the lens or camera is operating (e.g. shutter button half pressed, autofocus, image stabilization, continuous focus, live view, movie mode, etc.) can damage the electronics. Such damage WILL be expensive to repair and WILL NOT be covered by insurance. You have been warned.