I have an M 20mm f/4.0. It was bought used back in the eighties when I was a student. I had to dig deep to find the money asked.
Not owning a full frame body, this lens gets little use at present. Last year I tried it on my K-x for street photography but found out the hard way that the hyperfocal distance marks are wrong for a crop frame. Easy to rectify that mistake, but then I got a Samsung NX100 for that purpose.
For them moment, the lens is not for sale. This might change if the price keeps going up, but it is still unlikely. Even if I don't use it much, it rounds out my collection of Pentax glass.