Well, my mamiya is lighter than my canon DSLR, not smaller, just lighter ;-)
The best bit about the system is that since it has a focal plane shutter, you can stick all sorts of medium format lenses on the front, so I can use hasselblad lenses, mamiya 645 manual focus etc with little trouble. Mamiya 645 manual focus lenses are dirt cheap, so 50-100 quid gets you a large selection of reasonably fast, very good quality lenses.
The only real downside is actually that the sensor isn't sensitive, it's base ISO is 25, and it really only does to ISO 200...