Upload & Sell: Off
I own a D700, X100, Mamiya 7, and F100. The Mamiya 7 is my favorite camera of the four. I held a Pentax 67 recently, and while it was lighter than I expected, I was thankful in a new way for my 7. I own both the 80 and 65, and the 80 is perfectly balanced with the camera. It's also spectacularly sharp. The 65 is sharp as well, but the 80 is in a class of its own.
The only legitimate issue I can see someone having is the relatively small minimum aperture-- f/4. It's not an issue for depth of field in my view, but I don't share the obsession many people have with matters related to such things (the 150/4.5 is available if necessary, and it happens to be the least expensive lens on the used market). It is, however, an issue for light gathering, which is part of why I've begun using a flash with my 7. It's possible to make up some ground by shooting at a lower shutter speed, since it's a rangefinder and one with a leaf shutter.
I like rangefinders and find focusing with them easier than a manual focus SLR. Your mileage will vary.