The new generation of Canon TS-E lenses look great, especially that tilt/shift/rotate ability without removing any screws. Its expected that Nikon will upgrade theirs in the next fifteen years or so...
The reason some might use a 45mm and pan it versus a 24mm standalone is simply pixels and to some extent distortion on the wider lens.
I have the original three Nikons, but the 19mm is out of the question for now at $3,200. I originally bought the 24mm PC-E because I was frustrated with the towers in the desert, like Marlboro point, where no matter what they were always tilted. And then I ended up with all three because after the tsunami I was worried that I wouldn't be able to get them anymore, there was a shortage of many Nikon lenses - so I blew too much cash at that point in time to get the set. These days I'm now using the 45mm PC-E Nikon more than my 24mm or 85mm just because it works well for me. And I'm also not trying to go as wide as possible all the time. But that could change tomorrow. If I'm shooting landscapes those three lenses are usually all I take... It feels so light compared to that Sigma 150-600 Sport...
If you're into panoramas, and once you've used it, you'll begin to feel that the only proper way to create panoramas is to combine the lens with a leveling kit and then a quality panning platform from Really Right Stuff (Pano elements kit $380) or others (if any exist). I found using the lenses tilt/shift mechanisms standalone too cumbersome to create panoramas consistently and accurately. But that also depends on your own personal threshold for finger manipulation, little knobs etc. Personally, I would bump the tripod, or a knob would loosen, and the lens would tilt down completely ruining focus, lots of things go wrong. Then you get home and go "Arggg!" With the Pano Elements kit, it becomes so easy, you pre-frame your shot, remember the index marks on the base, and then just rotate the base to (for example) the 30, 60, 90 degree marks and fire off the shot. That way you don't touch the camera, the settings, the lens, or bump anything. However, since I'm also not shooting panos that much anymore, I use the PC-E and leveling/pano kit to keep everything perfectly level in the shot, and then compose it with the shift and the leveling base . I also re-aligned my shift and tilt onto the same axis since Nikon doesn't come that way.
It's kind of an expensive "investment" (Ha such a funny word) though if you don't have anything to shoot on a regular basis or a reason to use that type lens. So, I'm not telling you to buy anything - until your sure it's for you. And, you have to go through a learning curve to start shooting right. The below link is required reading IMO ! Sure, a PC-E or TS-E lens can be slapped on the camera, focused, and you fire away, but the learned skill becomes getting the plane of focus sharp and at much lower apertures, and getting creative with the lens. I now usually shoot at f/5.6 or even f/4 rather than f/11 or f/16.
One other thing too, that's a little important is that if you are not good at manually focusing, or your eyes are getting worse as we all age, these may not be the best lenses to invest in. Just saying...
This information is excellent from Cambridge in Color:
http://www.cambridgeincolour.com/tutorials/tilt-shift-lenses1.htm
http://www.cambridgeincolour.com/tutorials/tilt-shift-lenses2.htm
What was the question...?
Oh, buying a used one? Not sure about Canon, but with Nikon they always come up for around $1,000 less than MSRP and typically can be found brand new in the box. People buy, try, and then put on the shelf...
