This is Canon's newest fisheye lens, replacing the venerable 15mm f/2.8 fisheye in their lineup.
The most remarkable thing about this lens is of course the fact that it's a zoom; it was the first fisheye zoom on the market which works on full-frame cameras. It gives you an 8mm, 10mm 12mm and 15mm fisheye in one package, and the usefulness of that is illustrated by the diagram below from Canon's publicity material.
It's a well-built lens that delivers superb image quality, as you'd expect from any lens bearing the "L" designation. But it's also great fun to use. The minimum focus distance is extremely short - objects only about 2cm from the front of the lens can be in focus - which opens up huge creative possibilities.
If you're not quite sure whether or not this is the right fisheye lens for you, please check out our blog article "Which fisheye lens?" which illustrates the effects you get using different types of fisheyes on different types of cameras.