Summary: I had Contax 645 and Mamiya AFD and can say that now I'm with Pentax 645NII. I even bought 645N as a second body. AF is very fast for medium format cameras, lenses are great and electronics are perfect. Also you can buy genuine Pentax 6x7>6x45 adapter to use great 67 lenses on your Pentax 645!
Summary: Fantastic camera. The auto focus is fast enough for the type of work I do with this camera, mainly landscape photography. The metering sytem seems to be right on if you keep in mind that no metering system is perfect. I love data imprinting between the frames on the film and not on the image.
Summary: I have been shooting 35mm for a long time and always wanted to try medium format, but the cost of most systems kept me away. Then I got a great deal on eBay on an almost new Pentax 645NII. So far I'am more than happy with it.