Laudrup is right - I do have the 3 tilt-shift lenses, and a decent amount of experience with the Nikkor 24mm and before that the Canon 24mm before I switched systems.
I use the 24mm to shoot large products (like 6ft+ tall and wide), exteriors and interiors, predominately for converging verticals. I use them less for the Scheimpflug tilt effect, although I do use tilt more with the 45mm and 85mm (smaller products but requiring large DOF). I have also used them to shift on a horizontal axis to give a hi-res wide image composed of two frames simply by using the two extremes of shift.
There is no doubt the 24mm is a cracking lens, very sharp, excellent optics and very well built. Mine was bought from new from
Park cameras from the proceeds of the switch from Canon. The others were bought used - and as you say all come up rarely in the used market - it took me a very very long time to get a 45mm used. It is manual focus but the reality is that for the kind of work where you need a tilt-shift focussing is often one of the least time consuming parts of setting the shot up.
You can hire them, quite reasonably, but of course it depends on the frequency you use them and the urgency as to whether that is a viable route to take.
In the past I used to shoot my venue images for weddings with them taking a little more care when I had time, but now just use them for commercial work, having stopped shooting weddings. For weddings if I didn't have time then I did just shoot with a 24-70mm or 14-24mm and then sort out the verticals in PS. It does do it well enough for a wedding album, and again for that kind of product file size is rarely an issue - as you will lose some of the pixels as you correct the verticals in the computer.
The PC-E lens allows you to get the verticals right in camera - saving you some PP time, and crucially saves you all the pixels - so where image size is important - and in some cases my images are used at least 10-12ft wide on exhibition panels. That and the fact that is you are shooting an interior then you may need the full width of the frame for the image - and you may be unable to retreat further back to get a wider field of view if your back is against a wall
If I was selling my tilt-shifts then the 24mm would probably be the last to go as it is most versatile in use.....
Agree with Richard - I'd outlay on a hire and try it out before committing even on the lower priced Samyang.