This is the widest non-fisheye lens that Canon make. On a full-frame camera (1DX, 1Ds series, 5D series, 6D) it has an amazing 114-degree diagonal field of view.
The original EF 14mm f/2.8 L had been available for over 15 years, but the hugely improved resolution of modern DSLRs meant that its optical shortcomings were showing up. Hence the new improved version was introduced in 2007. It's physically similar, but optically far superior to the Mk I version. It's sharp wide open and even sharper stopped down; and it's impressively sharp all the way into the corners of a full frame image. The usual problems from which ultra-wide lenses are prone to suffer (flare, chromatic aberration, distortion, etc.) are very well controlled.
Its pricey, but that's perhaps not too surprising - there’s really nothing else this wide and this good for Canon users.