GX680 is an excellent platform for film. The large image size and the perspective control are very unique compared to other MF platforms. Be warned through, if you want to use it for digital.
Like other cameras without geared control, you need a very good skill for precise manipulation. It's very possible you don't. You might find the focusing is never right. due to the lens design, you could also find unacceptable color shifting and light fall-off in the edge. Digital backs without micro-lenses could help. Of course, you can also use SW to correct it. But that's not the same if compared to the lenses designed for digital.
One thing for sure, you'll like it to be in you backpack. I can assue you it helps to build a strong back,
I use the GX 680 with three lenses (65, 125 & 210) for the last three months with my Contax 645 fit imacon 528c back on it, via a chinese adapter. It has replaced my Sinar P2 work in stills and the painting reproduction I was up to now doing with my Contax 645 and Zeiss 120 macro combination in microstep 16x. The focusing is dead accurate, the lenses are as sharp as the Contax-Zeiss 120mm f4 macro, there is no vignetting whatsoever (how could it be on a 37x49mm sensor

). Color shifting?

The lenses are so neutral that they beat the Zeiss in painting repro, the profile down to the final print needs far less fine tuning now, with the Fuji on microstep you won't recognize the 1:1 size print on 390 canvas from the original oil on canvas painting, if you will stand only a yard away from both, no matter the size of the original! Backpack?

What do I have the Contax (3 bodies) with all lenses but the 350 for?

Cheers, Theodoros.
www.fotometria.gr