I recently had the opportunity to do a short lens swap with a fellow Canon owner. He got my EF L 17-40mm lens and I tried out his 90mm tilt/shift lens.
For those of you that have never used one, the shift function allows for adjusting perspective. It is especially useful for architecture as it is rarely feasible to have your camera perfectly level so perspective comes into play. Architecture usually requires wide-angle lenses which creates even more perspective distortion. By shifting the lens parallel to the camera perspective can be either removed or accentuated.
The tilt function tilts the lens from the camera plane resulting in the center plane in sharp focus while the top and bottom are blurred. The lens can also be rotated 90 degrees to give a sideways depth of field simulation. It is sometimes used to simulate a miniature scene. I did a Photoshop simulation of the tilt function for my Grand Central Station picture on my Project: 90 Days site.
I'm not really sure what the practicality of a 90mm tilt-shift lens is, especially on my 20D with the 1.6 crop factor. When I went out to shoot, I had to shoot everything from far away. I mostly used the tilt function because the shift works more effectively when the camera is on a tripod.
This image of the taxi cab was my favorite one from my little tilt/shift excursion. It's a little bit iconic with a personal twist thanks to the lens swap. It ended up as my photo of the day for my Project: 90 Days.