I've been using the Pentax 645D for about six months so far, and very much liking it, mostly for landscape work. Photography is a hobby, so I can't speak as a pro, and I've not used any other medium format systems to offer a comparison. My other system is Nikon (D700) with a full assortment of lenses, including the ZF primes. What tipped me to the 645D were a couple of points which aren't frequently mentioned: with a few adapters, the selection in glass can include Hasselblad V lenses CF, CFi CFE, Hasselblad F & FE, Pentax 67 and Pentacon Six lenses. There's a lot of great glass that can be put on the 645D, but it does take time and patience hunting down good clean copies. Some of them are down right bargins (although prices have spiked for the 645 lenses in the last year.) It isn't for everyone, and the manual focus (& manual exposure) make for a slower workflow. I find it works for me, for doing landscapes, cityscapes, and still life. The 645D does have a nice feature for exposure -- instead of spinning the front finger wheel to adjust shutter speed in manual mode, you can press the green dot button, and the camera will immediately set shutter speed to what it thinks is correct exposure for the current meter mode. You can then adjust from there with the finger wheel, if needed. This makes manual exposure much faster, especially when using the lenses that require manual stop down.
With dual SDHC slots, the 645D works well with an EyeFi card in the 2nd slot, so it is possible to wirelessly beam 10mpix jpegs over to an ipad for preview in the field via Shuttersnitch photo application. It isn't instant (10 - 20 seconds) but being able to view on a larger ipad screen is quite handy. The EyeFi appears as a WiFi hotspot so the ipad can directly connect without an internet connection. The full size 40mpix raw image is written to a regualr SDHC memory card in the 1st slot, to be manually downloaded later at home into Lightroom. Battery life, even with the wifi card, is pretty good -- good enough that I usually can get by with 2 batteries (in mild climate). The build and ease of use on the 645D is first class, and other than a few small differences, going from a D700 to the 645D took about a day or two to adjust. The 645D can do long exposures well -- I've done 30s-4m exposures in bulb mode, no problems. I normally use base ISO of 200, but going as high as ISO 800, the noise hasn't bothered me much (as long as you don't push too much in post).
I do wish that someone would make an adapter with mild tilt capabilities. I've seen the threads on modifying a 67 55mm using shims, but haven't tried it yet. The depth of field difference between the 645D, the FF D700, and the crop D300 is very noticable, and I find getting everything sharp requires a lot more attention (or, if subject allows, use multi-shot techniques and use focus stacking software...)
The Hasselblad PC Mutar shift adapter does indeed work on the 645D w/ 645-HB adapter, but it also adds an optical 1.4x tele-conversion.
Some examples are up here:
http://www.flickr.com/photos/groggylobster/The lenses that I've tried so far on the 645D (all manual focus)
The short list:
SMC Pentax-A 645 1:3.5 35mm
Hasselblad / Zeiss Distagon 1:4 40mm CFE IF via adapter
SMC Pentax 67 1:4 55mm via adapter
SMC Pentax-A 645 1:2.8 75mm
Hasselblad / Zeiss Planar FE 1:2 110mm via adapter
SMC Pentax-A 645 1:4 120mm Macro
Hasselblad / Zeiss Sonnar CF 1:4 180mm via adapter
SMC Pentax 67 1:4 300mm EDIF via adapter
I've also used these on the 645D, with no issues:
Hasselblad / Zeiss PC Mutar 1.4 (CF) (w/ 50mm CF FLE)
Hasselblad / Zeiss 50mm CF FLE Distagon
Hasselblad / Zeiss 60mm CF Distagon
Hasselblad / Zeiss 100mm CF Planar
Hasselblad / Zeiss 120mm CF Makro-Planar
Hasselblad / Zeiss 250mm F Tele-Tessar
Hasselblad / Fujinon 60-120mm 1:4.5 FE Zoom
Carl Zeiss Jenna 180mm 1:2.8 via Pentacon Six - 645 adapter
SMC Pentax-A 645 45-85mm 1:4.5 zoom
SMC Pentax-A 645 80-160mm 1:4.5 zoom
SMC Pentax-A 645 135mm 1:4 LS
SMC Pentax 67 165mm 1:2.8
SMC Pentax-A 645 150mm 1:3.5
SMC Pentax-A 645 200mm 1:4
*) To use the Hasselblad CF, CFi, CFE lenses, cock the lens if it isn't already, put on the PT645-HB adapter, mount lens + adapter to camera body, set the lens' shutter to use focal plane, open the lens aperature with the stop down lever, select desired aperture on the ring, focus while wide open, stop down aperture with the lever, set exposure, take the shot. When using the adapter with a Hasselblad lens on the 645D, there is no electrical or mechanical linkage between the lens and the body - the lens will not fire and should remain cocked. You're using the shutter in the camera, and stopping down the aperature on the lens manually. The Pentax 67 lenses are a bit more convienent -- you don't need to manually stop down, just select aperture with the ring on the lens, focus & expose, take the shot. In the case of 67 lenses with the right adapter, there is mechanical linkage so the camera can stop down the lens. The 645-A lenses have cpu contacts, so you can select both aperture and shutter speed with both finger wheels on the camera body. I haven't tried any of the 645 autofocus lenses. The 67 & Hasselblad adapters are from a company called Fotodiox. The adapters for Pentacon-Six to 645 can be obtained online from a few distributors in the Ukraine.
