I was excited to hear that others love their MZ-S. I recently acquired one on eBay for $600.00 (body and BG10 only). It is in near-mint condition and I already have another so I was worth the price (about 1/2 what I paid in 2000).
I also own a Nikon F80QD and I must tell you, the Pentax MZ-S is vastly superior to that Nikon. Most of my lens investment is in the PK type so the Nikon only gets used when I use my amazing wide-angle Nikor lens (have yet to find a wide angle as good as this).
I will admit that the plastic back on the MZ-S was a little disconcerting, especially considering that the rest of the camera is so solid. Why? But on the bright side, it is easily replaced (simple removable hinge) so.... I think it is a good idea to have a spare, although I have owned one MZ-S since 2000 and have taken over 2000 photos from around the world. It's been banged around a lot and still performs flawlessly!
Still, there is a LOT more plastic on my Nikon F80QD and I think I paid more for this camera.
I noticed a few are looking for the MZ-S. I will keep an eye out for another when I search eBay for any MZ-S and K20D accessories. Specifically, I would love to purchase a digital timer (preferably one that works with both cameras).
Best,
Doug