Having owned and used two 645 medium format camera, the Contax 645 and the Mamiya 645 AFD which runs on film, the Pentax 645Z would be my very first digital medium format camera. Hence to be fair, I could only review and comment on Pentax based on the ease of use, ergonomics and features .
Starting off with the overall built quality, I find it to be really sturdy or at least on par with the other 645 contender in the market. The button placement is very well thought off , necessary buttons are all placed near the thumb and index finger while less frequently used buttons are placed further away to avoid accidental activation. The swivel LCD screen is really great specially shooting in some of the hard to reach angles. I love this feature a lot! The experience of using this camera reminds me of some of the old Panasonic prosumer camera FZ-50 which I had used over the years. The size and weight is pretty much the same as other make in the market.
I think one of the best selling point of this camera is the ISO capability. I am able to get decently clean shots at 2000 to 3200 ISO which I feel it's amazing for a medium format. It is still able to retain a lot of details and sharpness even at such high ISO, thanks to the new CMOS sensor. This opens up a lot of possibility to a lot of photographers as the camera need not to be glued to the tripod at all times ! I have a feeling that wedding photographers will enjoy this camera a lot :) I really like the neutral color rendition from this camera. I shot only jpeg with this camera and added a mild touch of personalization on the images attached.
The autofocus on this camera is rather fast and accurate, way better than my Contax 645 but again I am comparing this to a 15 to 20 yrs old camera which might not be relevant. The mirror slap could be a tad louder than I expected but I could bear with it. I am able to steal some shots taken at low 1/60s which fairly acceptable sharpness.
While on the lens, I opted of the SMC Pentax-Fa 75mm f2.8. The lens is really sharp even when shot wide open at f2.8. I can't find any fault with it at all but I still prefer the Zeiss Planar 80mm f2 on my Contax which I feel it has a little bit more character. Probably because I shoot wedding and I love the more dreamy feel on my wedding images, the F2 really shines. I think for those does commercial, the Pentax lens would be really awesome. I really like this camera a lot for what it's capable but I still heart the Contax 645 more as film is closer to my heart :)