This lens gives a very speciallized trade-off: you gain perspective correction and ability to tilt the focus plane, at the expense of moderate aperture, heavy weight, large size, and high cost.
Perspective correction is, I think, a necessary step in any photo contain a building or interior. (That is to say, sides of buildings and rooms should be square, not converging lines.) Before computer editing, that meant the camera ABSOLUTELY HAD TO BE aimed perfectly level. If the building went too high (or, in an interior, you wanted more floor and less ceiling) you had to either 1) use a wider lens and crop, or 2) use a shifting lens such as a Canon TS-E.
However, once I started scanning film and editing on the computer, I saw no point in using a shift lens any more. Thats of course even more the case with digital bodies.
Tilting the focus plane is theoretically an excellent feature: it lets one side of the photo be focussed near, and the other side far, with complete control over how near and far. If the image in-camera is not in focus, no amount of editing will make it in focus later. If the lens has nice blurring of out-of-focus areas (called "bokeh"), you can even tilt the opposite direction to LIMIT depth of focus (DOF).
However, at 24mm and with f/3.5, the DOF is so large that one almost never needs to increase it, and there's not enough bokeh to artistically exagerate either.
One additional minus is that as you shift, the vigetting is very strong; fully-shifted corners may be 3-4 stops darker than center. (Canon should feel free to edit this and put the exact figure in.)
A second problem is that as you shift, the center of your photo is no longer at "0" on the MTF graph with excellent contrast and good detail. Instead it moves towards 11mm, and the far corners are no longer at 24mm (the right side of Canon's MTF graph) but rather off the graph at 35mm. This means that both detail and contrast fall away rapidly.
In summary this lens should most likely not be considered unless one is still working with film (without digital processing), and wishes to print or publish exactly what was in the camera without cropping. Otherwise, you'll have a much lighter and more flexible camera bag with a 14mm or super-wide zoom, and crop shots aimed perfectly level, or use some other 24mm solution (standard zoom, lightweight 24/2.8 or whatever) and correct perspective by stretching the photo as needed during editing.