I recently was able to get my hands on both 24mm and 45mm PC-E lenses (on DX). With perspective control, one can modify the focus plane (tilt) and image circle (shift).
I was able to spend several days with them, and wound up buying the 45mm version. Here is one of the shots taken with the 45mm f/2.8, handheld:
Here is a crop of the horizon on this shot: Remember, this is 2.8 handheld!
I tried to get the focus plane as horizontal as possible in this shot so that it would match the road to horizon.
If anyone has some experience with tilt and shifts, or would like to talk about it, feel free to comment!
By the way, if Gordon is reading this, I'd like to say that I loved the Nikon 50mm f/1.4 review, which is what got me to share this.
Here is another image taken of Santa Monica, California hillside: