Back in the 70s when I was using the Contax RTS system, I bought a Mamiya 645 and was so excited by the quality of the images but did not pursue MF further as my work was in gymnastics and I needed the Zeiss fast lenses to compensate indoors for shooting without a flash. GAF500 (grain the size of golfballs) and Ektachrome pushed a stop were my staples). Now that I had less need of f1.2 and 1.4 lenses, I was tempted to get a Mamiya 645 ProTL and a few lenses - I was hooked. I then graduated to the Mamiya RZ system partly because of the larger format but also because of the quality of the glassware and sold all my Mamiya 645 kit to help pay for the 'upgrade'. I was perfectly content until ebay intervened and one one night I bought a Contax 645 (old marque loyalty) with 5 lenses and then followed that up the same day with a Fuji GX680II whne MrCad had a half-price sale. Again, for me it's the glass that counts. Although the range of lenses on the Contax is limiting, their quality is breathtaking and I now understand why their s/h remain obscene - espcially in the US. I also love the ergonomics of the system - reminiscent of the quality of the original RTS with its miriad accessories.
However, for me the Fuji was the revelation. After shooting just 2 rolls of Fuji 800ASA (sadly gone for ever) through the basic 135 f5.6 lens, I was hooked. Checking out comments here and elsewhere, I bought 2 GX680III and 2 GX680IIIS bodies and every lens from the 50mm to the 300mm including the 100-200 zoom which is superb. The only lens I don't have is the 500mm as I can't see a use for it in the studio and the 300mm is long enough for my outdoor needs. Adding the AE Finder and the Magnifying Hood (a brilliant viewing accessory) has given me a lot of shooting versatility and of course, the beauty of the Fuji is that it can shoot 6x4.5, 6x6, 6x7 and of course 6x8. Add to that the GX680III has a full set of tilt, shift and swing movements that can be used with all but the 50, zoom and 500mm lenses and you have the ultimate MF system camera -admittedly though, it is a bit weighty. But the IIIS is lighter and less bulky and is the ideal partner for the 50mm super-wideangle.
Of course, this has rendered my Mamiya RZ67 kit redundant - I'll keep the Contax 645 for its greater ease of portability - so I'm going to sell all of it bar the ProII and 4 lenses. I can't justify the space for all this MF gear. Oh yes, I know it's not a system camera but my one remaining and oft-used MF camera is another Fuji - the delightful GA645Zi which is the perfect travel companion - not a phrase likely be in the same sentence as a Fuji GX680!
And another thing - though heresy in this forum - I have a superb old Ixpress 384 digital back with adapters for the Contax, Mamiya and Fuji cameras for those moments outdoors when the film runs out.