Eric's answer makes a lot of sense - I came across him about a year or so ago shooting a building in San Francisco. I was with a digital Leica, him with his Alpa on a tripod. After discussing the joys and concerns of LF and digital (how to get a good back, addressing centerline and white balancing issues, the need for a good dealer to stand behind the products), what impressed me is how the Alpa and back really did replace the 4x5. Eric could pick up the entire camera/back/tripod setup with one hand, and walk around with it, easily. In the streets of San Fran, it seemed a bit nerve racking to have that portability, but boy, was it light.
Fast - not really: each shot required test shots for accurate composition, but you could do it all in the field: focus checks, exposure, and whatever else you wanted. As to crispness, and definition- it was exemplary.
There is probably something still outthere between handheld smaller digital gear and this kind of setup - perhaps architectural quality compositions can be achieved by MF gear (Hy6 anyone?), but for detailed work, the Alpa and backs give a real quality boost, while still very portable. Its sort of like 4x5 work that you can easily carry around and use. There are lots of little steps for each shot, so its not for thoughtless work, but great results with such a handy setup.
Geoff