I had a Pentacon Six, sold it and bought a Pentax 645N.
For me the P6 has the following disadvantages:
- viewfinder does cover only a rather small portion of the frame; with prism it is even worse
- the shutter and mirror produce quite hefty vibrations
- the tripod connection is a bad joke
- the screen is plain matte and dim
- robust feeling but fragile inside
Some of these can be solved by upgrade e.g. at www.baierfoto.de, others not.
The lenses are reported to be quite good; but the widest (Zeiss) is a 50 and this one is rather expensive, heavy and bulky. This is valid for the 180 as well.
I sold it and never looked back.
P6 was a good system when more modern MFs were much more expensive. Times have changed.