Hi there candyman, I use 11mm and crop, I have found it very useful for framing loose to allow for the inevitable lens corrections and straighten and level that always seemed to cost me too much fov.
Also the extreme fov also allows for the lens to be used as an effective shift lens with better edge characteristics than the TS-E17 when shifted. For example the image here http://www.canonrumors.com/forum/index.php?topic=24975.msg529051#msg529051 was shot at 13mm but framed in the top left hand corner with the camera close to level with the intention to crop, this kept the flagpole vertical and gave me the framing I wanted, the shifted 17 is not as nice in the corners, though it does have other advantages.
Here is one from last night, small town USA. 24mm, f13, 60sec, 100iso. This lens handles light sources in the frame very well.