If you use one of the 1.6x image sensor Canon camera you are better off looking at Canon's 10-22mm zoom that provides true wide angle coverage for those cameras. It will save you some money and provides the equivalent of a 16-35mm zoom for that sensor size.
The version II of this lens is much sharper than its predecessor and is a great ultra-wide lens for the Canon pro bodies (14mm on the 1Ds Mark III and 5D full frame; 18mm equivalent on the 1D Mark III)
Not worth the money for 1.6x sensor camera bodies (i.e.; 20D; 30D; 40D; Rebel XTi; etc...)