I shot with it at Photokina this week, and I was very impressed with just about everything the camera offers. The menu / LCD, handling, shutter slap, autofocus, etc etc. To me the proof is in the pudding with the file quality. I think the Hasselblad H4D 31 pricing is going to seriously affect the viability of the 645D, primarily because of the installed base and known quality of the lenses. I haven't seen a Pentax 645 lens on a shelf in quite a few years, and their marketing online is very slim with regards to what is actually available.
The 645D has more DNA of a 35mm DSLR than a medium format camera, and I think that is a good thing.