The 645N or 645NII are more functional than the original 645 (I agree with the previous poster); all of these cameras, however, are extraordinarily reliable. Apart from the 645 diopter, better on the 645N, I haven't heard of any errors. I'm sure there must be, as with all cameras, but these are rugged ones. And ergonomic too—you hardly believe it until you use one. I find AF good enough for my purposes.
And the lenses are tops, at least most of them.
Above all, these cameras and lenses tend to keep their value. So you can try them out and if it's not for you just sell them on again.