My experience with handholding the mamiyas is quite different from what Luis reports. I have plenty of printworthy shots on the 6 taken at 1/8 or 1/4 and even slower. There is a simple trick for going to long handheld exposures: use the timer. That removes the finger impulse which permits you to focus on stability. Try it.
That's fine for static subjects, not for what you'll do with the leica - let's say - in a jazz club with moving targets @ ISO 400, that's my point, other subjects, other rules BTW, I'm a real nitpicker for sharpness.