I'll reign in my initial reaction and just say that I respectfully disagree with you.
Originally Posted by Tom1956
645 is something like double the image area of 35mm (I'll let someone else do the math(s) if they want to get a precise number) and whatever those numbers may seem to imply, the actual impression of step up is substantial. It's also the same image area you will get if you crop a 6x6 negative to 8x10 proportions which I find I do about half the time with my 6x6 negatives (the fervent anti-cropping crowd can disregard that.)
It is true, of course, that 6x7 is a huge step up again, doubling the negative area again. But it's also a huge step up in size and weight. A 6x7 rangefinder will be huge compared to a 645 one. There are folders but any camera in 6x7 is just going to be a lot larger than the same type of camera in 645. An RB 67 makes a great tripod camera but, while it CAN be handheld, I wouldn't swap my Mamiya 645 Pro for one for carrying around - the 645 Pro, as someone alluded, is big enough.
The 645's proper competition is 6x6 not 6x7. And it just depends on whether you will crop much of the time, in which case you might as well shoot 645, or print square. In this case we are talking about slides and a projector that can handle 645 and 6x6 will be MUCH more affordable (though still a lot more than one for 35mm) than one that can handle 6x7.
I shoot all three in one form or another, 645 in my Mamiya 645 Pro, 6x6 in my Yashicamat 124 and 6x7 via a RF back (mainly for color) in my view camera. Leaving aside the view camera which is a very different kind of shooting, the Mamiya is the most versatile, the Yashica is the most fun, and 35mm does in fact often get the nod over the 645, partly due to size/weight but mostly due to fast primes and having a couple of good zooms in 35mm. The 80mm 1.9 is, of course, available for the 645, I just don't have one.
I will agree that a 645 Pro is a big honkin' camera to carry around though. Even with the 80mm 2.8 or my 55mm 2.8, add a film magazine and the winder grip (which makes the camera handle MUCH better) plus the AE prism finder and it does somewhat resemble an albatross around one's neck. It handles great though and the results are superb, and the size/weight penalty relative to 35mm is often mostly offset by the interchangeable backs meaning I don't have to also carry spare bodies loaded with different film. This may may not matter to you but I often need at least a couple of kinds of film available and loaded, even with the 15 shots per roll I get with the 645.