I am currently researching these lenses to add a new perspective to my portfolio (forgive the pun).
The reason I am interested in them is multifactorial .. I originally thought about them exclusively as architectural lenses, used to produce straight parallel lines, which is of course true. But then I realised they have so many other uses and will therefore rent one to try out, before I actually make a plunge and buy one.
Uses as I understand them, and please correct me;
1. Controlling perspective
2. Controlling DOF; sharp focus across the image, while still using a wide or average aperture to avoid diffraction
3. Controlling DOF; selective focusing reduced to a small area, thus producing a miniature effect or pleasant BOKEH
4. Ability to shoot panoramas easily for later stitching in software
For these effects I decided to try out the Nikon PC-E 24mm f3.5 lens. Some but not all the effects can be reproduced in software, but not always very convincingly natural.