I sold my 645 system(Pentax 645nII) but have been shooting some 35mm recently, and in all honesty, there is a HUGE jump in the quality and "draw" compared to using 645, much less 6x7. Almost to the point that I've considered selling my Nikon system and going back to a Mamiya 645AF or something like that and leaving the "miniature format" stuff to the digit@l realm entirely...

That said, I also use a GX680 kit, and despite it being heavy and bulky, the lenses are stupendous, and absurdly sharp even wide open.
And the 3x4 ratio(same as 645) is nice too.