Thanks to an offer I couldn't refuse I've owned a 50c digital back for a few weeks now, and overall am very happy with it!
This substantial upgrade did however poke a big fat finger at my inadequate lens collection. I vividly remember when I got the CFV-II (the 16MP back) about 6 years ago, I was absolutely astounded at how crisp and rich in contrast and sharp the images looked. That was of course due to the near perfect matching of the pixel size (12 microns from memory) with the optical system. My fear when upgrading now to the 50c back was that the 5 micron size pixels would be recording at a far too high resolution. It turns out that for some of my lenses this is true, but for others, it isn't.
So I've started weeding out the bad and upgrading the essential. In summary, the CFi 30mm and CFi 180mm provide excellent image sharpness and crispness with this back, almost as good as that first back did. I further own the CFi 120mm (mediocre), CF 80mm (bad), CF 50mm (so so), the way old Synchro-Compur 250mm (not as bad as I feared) and the 500mm Apo-Tessar (surprisingly quite good!).
But out of all of those lenses, the 50mm is really the one I use the most so am wondering whether I'd get better images by upgrading to the CFi. Has anyone done a comparison of the 50mm CF vs CFi? The Zeiss documentation seems to indicate that their MTFs are identical. Is that true?