I use Fuji GX 680 with 65, 100, 125, 150 & 210 glass, 3 finders (WLF, loupe & prism) film back and adapter plate for Contax 645 MFDBs... For backs, I share my Sinarback 54H & Hasselblad CF-39MS between my Contax 645 system & the Fuji. I am very pleased with the performance of the glass I use and using a Contax 645 back on it is completely trouble free, as focusing is dead accurate and focusing to infinity is no problem. Additionally, the backs I use need no "one shot" cable and more than that, I can have very good LV with it by using Sinar's LC shutter on it with the Sinarback 54H.... The image quality result on stills when using the Sinarback 54H in 16x mode is simply "jaw dropping"... a real world apart than anything a 60mp back or an 80mp one can ever achieve...
The system's limitations are the following:
1. It doesn't cope well mechanically with Hasselblad H backs, so H backs are better to be avoided with it....
2. It doesn't sync well with backs that need "waking up".... so backs that need an one shot cable are better avoided
3. One can't go wider than 50mm with the lenses... So if he is to use the full potential of its 6x8 image area, one has to either use film, or a stitching adapter, or a scanning back on it....
4. A stitching adapter is only available for Hasselblad V mount MFDBs, so one that uses interchangeable plate MFDBs (like in my case) has to change the back's plate to Hasselblad -V as to attach it on....
Other than the above, the 50mm lens has less wide image circle than the rest of the lenses and it won't allow full shifts if the whole 6x8 image area is to be used... thus, it is alo expected to be the softer one near the edges and corners when no movements are applied... but it is a great lens to use with an MFDB (with movements too) if no stitching is required...
IMO, Fuji is a great system (especially if one considers the cost of the system) provided that one doesn't need ultra wide AOV or "one shot" cable, or the back is of H fit...
P.S. For digital (especially for use with multishot backs), the most "quiet" out of all Fuji GX-680s is the (I) followed by the (II)... There is absolutely no reason for one to pay extra for a (III) body unless if he intents to use a film back of 6x6 or 6x4.5 with it where he will lose the WA coverage any way...