The tradeoff between bulk and negative size is really always a personal call. Some people find the 645 SLRs to be too bulky as walk-around cameras to justify getting out of 35mm, others don't; by the same token, some people are OK with carrying around one of the big 6x7 cameras, others aren't. My dad used to know a guy who regularly went mountaineering with a Pentax 6x7 kit!

Personally, I've found I don't like the 6x7 format very much, and 645 SLR + 6x6 TLR pretty much covers my medium format needs. (Though I'd like to find a 6x9 back for my Optika.) But there are people who think 6x7 is just the cat's pajamas, whatever the hell that means, and you may need to try it out to determine whether you're one of them.

The various Mamiya 645s are so darn cheap that I think there's no point in *not* buying one. Try it out, see how it works for you, and go from there. If you don't need removable backs, the bodies are practically being given away.