I have the GF670 for about 1.5 years now, and its a stunning camera.
Before I bought this, a few other cameras were shortlisted based on 6x7 format as well as price. The Mamiya 7 & 7ii were one of them, but due to the price and size of the camera, it lost out to the GF670. The strength of the Mamiya 7 is the ability to change lens, but, the GF670 unassuming size, portability and ability to switch to 6x6, beat the Mamiya 7.
The view finder of the GF, built in meter (yes M7 also has it), but the GF has a better reputation for reliability.
Prints on 12 x 16 from the GF are superb, and I would rate this camera 10 / 10 if you are looking for a 6x7 format for travel or daily carry.