Back when it first came out I bought the entire Mamiya 6 system and dedicated one of the bodies to a NPC ProBack for Polaroids. I thought I was valued for my vision and spontaneity (rangefinder Leica ethos) but the unfortunate reality was that a medium format SLR was a lot more versatile for general commercial/corporate photography (graphic close ups of widgets for annual reports).
The Mamiya 6 with the 50 and 75 lenses is great, the rest isn't worth bothering with. If I wanted a medium format now though I think the Fuji 6x9 series is the best all around.
They are both dirt cheap (under $200). I shot a GX7 and it was nice but it seems that the GX1 is 98% the camera and smaller without the EVF (same or similar 16mp chip). I thought I would prefer the EVF but in both models it creates a lot of bulk that catches on your pockets, whereas the GX1 w/o EVF is quite compact. Then after using it all Winter I found that for most of the kinds of quick shots I do, I don't need or want the EVF. Mostly it is a pocket camera because I can close it up in a jacket and avoid condensation when I come inside from the cold. With larger cameras I am always using plastic bags to wrap them up and it's a pain.
The D200 with the 50/1.8 G is quite a nice portrait camera, I leave a 24/2.8 on the D300 for general purpose. I liked the look of my older portraits at low ISO with the CCD chip so I'm giving it another shot, at least for anything under ISO 400. Admittedly the D300 (or later) cameras are better general purpose shooters and if I decide that I'm not loving the look of the D200 then I will probably just buy another used D300 since it appears Nikon will never make another good yet reasonable solid DX body again.