I've never been attracted to the 645 format. It shows some improvement over 35mm, but if one has a 35mm system, I think a better move is to go to 6x6 (if you like the square) or 6x7 if you prefer a rectangular format. You get more bang for the buck and the bulk at 6x6 or larger. Even if you don't print square (though I usually do) from 6x6, you get the option of cropping from anywhere in the frame, so if you take the image from the top of the frame, it's like using front rise on a view camera, and if you crop a vertical from the left or right, it's like getting shift.

I would only look into 645 if I were going to have one system that had to be a compromise between 35mm and a larger medium format. The one possible exception might be the Bronica 645RF, if I wanted a compact rangefinder with interchangeable lenses.