The unfortunate corollary here is that there is now little added value of the Phase and Hassy 50MP Cmos solutions except for people who need hi sync speed. With the Sony or Nikon and a Sigma lens you get the same sensor technology with a slightly different aspect ratio and 20% less pixels for 1/10 price of the Phase. Sharpness is probably equivalent, and decent AF is thrown in 
At some point we're all going to agree that the emperor has few clothes; personally I think the low-rez CCD backs may still have something going for them lookwise, however the crop-CMOS Phase and Hassy solutions are nice additions if you are already in those systems, but don't make sense if you have zero lenses. The equation may change if decent-sized CMOS backs arrive, and we see some real MF again.
Edmund
The big issue with the Pentax is the lens selection. It is limited. If all one needs is satisfied by what is available then this is a non issue since most lenses are good at f8-f11 and a lot can be had for cheap. With the Nikon and The Sony A7R one has a LOT of different lenses available from extreme wide angles to very fast lenses to extreme teles and of course a selection of Tilt Shift Lenses.
Medium Format Digital Backs offer an amazing variety of lenses available when one includes SLR lenses (which are somewhat system dependent), tech camera lenses and also the 35mm SLR lenses that work with the backs using a system like the Alpa FPS.
As of today nothing matches the edge to edge performance (even when using most of the large image circle) of the current crop of tech wide angle lenses. So if wide angle work is your thing then tech wides are still the best option available in regards to optical performance. It is just stunning.
For normal and slight tele focal lengths then one start getting into the SLR MF systems and the 645Z and D810 are thrown into the mix but still the MF systems from Phase/Hassy/Leica offer the options of flash sync at all shutter speeds and of course the tech camera systems offer the option of in plane back stitching since most lenses in that range have very large image circles.
For tele and extreme tele work the 35mm DSLRs are still the best choice.
For studio still life and macro nothing offers the overall control of a tech or LF camera with movements.
But as usual YMMV and its great to have many choices since ultimately camera system choice is a matter of preference.
I for one love the big sensors and the image quality I get from my IQ160 is just awesome whether I use it on my Hasselblad H or on my Arca. The large file size combined with the physically large sensor just behaves very differently from the 35mm DSLR sensors. Having owned the 645D the difference in size is noticeable between it ad the IQ160 just as the difference in size between the 645D and the D800E is too. As the numbers suggest the difference between the IQ160 sensor size and the D800E sensor size is quite significant, less so when you jump from the D800E size to the 645D (and Z/IQ250) size but the difference is there and it is noticeable.
I also really like the PhaseOne IQ chasis. The screen is awesome, tethering robust and the back just feels very solid. Disregarding the particular sensor it houses the IQ platform is superbly designed and is really EONS ahead of the previous MFDB designs and even the latest H5D backs. Yes we all wish they were lower in price but unfortunately no one else has come up with a product that matches it.
Honestly just like I found the 645D to be in some middle ground that makes it a tough choice (for me at least) the 645Z is too. Nestled between the great Nikon D800/D810 (with its huge Nikon / Zeiss lens lines) and the MFDBs with the versatility of being able to use them in a wide range of systems / setups including tech cameras and their superb lenses.
