Just wanted to add. i went with a rb67... i do carry it around with three lenses (50, 90, 180) and i'm very happy. when i look at a hassy, only ever on a shelf at a store, all i can think is how minuscule it looks. The RB does weigh a ton, and the hasselblad looks tempting... but you know what... i love the large negatives, and no need for cropping. you could print square pictures, but like people have mentioned in other places, you then have to look for the frame, although i think there are more now than before. Even people who shoot with 6x6 end up cropping to 6x4.5 or thereabouts... and so you might as well pick up a 6x4.5. The weight might be a serious issue for some, i especially like to have the availability of changing lenses, and so carry all 3 lenses with me. I also carry a tripod... and although i've shot at slow speeds with the mamiya (handheld), i usually never do (i use the tripod). Price, is also a major thing... i got the whole set (3 lenses, 2 backs, metered prism, polaroid back, bracket, pelican case) for less than $400 shipped on the bay. i can probably get a hassy body for that price. i wouldn't worry about optics, for the sole reason, that even if zeiss glass is 'better', the working negative becomes half the size of a 6x7 (unless you're doing square), which kind of defeats the whole purpose (I think).
all in all... weight (even if you love the system to death, if it's to heavy, you just want to stop where you're at and get the shot from there)
price (if you really want the hassy save up for it, i'm sure it's worth it)