The 645D has changed the landscape of medium format digital. Pentax has given us the best handling MFSLR at the lowest price thus far. Image quality is excellent, and the high ISO is very usable. The Mamiya DF and Hasselblad H bodies have suddenly started to look very long in the tooth. All is not perfect, because Pentax really needs to release a full line of digitally-optimized lenses. The 55mm SDM is but a start, and a few question marks hover over the quality of their 'legacy' lenses.
All in all, however, Pentax has done a good job. The 645D is a photographer's camera. And that's the highest praise there is.