I have owned Contax, Hasselblad and Mamiya/Phase One cameras and lens systems over the years.
I have owned and shot with the P25, P45, IQ 180, and tested most other Phase and Hassy backs. I have also worked with the Leica S2.
In my opinion the Pentax 645z not just meets the others in terms of image quality, but surpasses most of them. It's hands-down a more user oriented camera than any of them, except the Leica S.
Leica S lenses are no doubt superb, as are some of the new Phase / Schenieders. But The six Pentax lenses that I own are excellent, and for real world photography (not lab tests or pixel peeping) they are generally excellent and a joy to use.
If one is a measurer, then arguments can be made. If one is a photographer, then I'll stack the Pentax 645z and its lens line up against anything on the market.... and if price is taken into consideration then there's a mid-range Mercedes or BMW in-between putting together a Phase One or S Leica system and one from Pentax.
As for the picture quality being "cheep", I'll be polite and simply call bullshit.
I'll take the Pentax + a fine car, thank you very much.
Michael