Yep, I moved from a Mamiya ZD to a Nikon D3x.
The files of the ZD were good, although not significantly better than those of the D3x.
But the main reasons why I moved back to 35 mm were:
- usability in the field for landscape (cold weather shooting battery life, speed, screen,...),
- disappointing lenses compared to the best 35mm lenses, especially on the wide side where the Mamiya/Phaseone 28mm f4 didn't impressed me much on the 48x36mm sensor of the ZD. Very sharp in the center but less so in the corners,
- overall reliability of the system (loose mount tolerances, some software bugs,...),
- bulk and weight indeed,
- lack of DoF when shooting at best possible aperture,
- lack of T/S lenses,
- lack of eco-system (brackets, raw files support in DxO,...).