I have a M645 (the old one, no interchangable back) and like it very much. No negative experiences so far. Lots of used lenses at reasonable prices to choose from. The 645J is a somewhat simplified version of the M645, no mirror lockup, IIRC only one shutter release where the M645 has two, and probably a few other details I don't recall... The importance of mirror lockup has been discussed at length, but I feel it's a useful feature for this camera. So my advice would be to look for a version with MLU; ie. the M645 or the M645 1000s.