Any time you move a mirror that large quickly, there's bound to be some noise. (Personally my RB67 - the Pro-S version - is quieter than my Bronica S2a which is a 6x6 camera.) Having said this, I find the mirror dampening on the RB to be extremely effective. I've shot quite a bit hand-held with mine (something that's best avoided if possible, but sometimes you have to do what you have to do...), and have never detected a loss of sharpness.

Like jdef, I can get away with 1/15th pretty regularly with no noticeable lack of sharpness. The "trick" that I use is to put a wide strap on the camera, put the strap over my neck, then pull down a little bit on the camera while pulling up a little bit with my neck. This seems to give me a very stable platform for hand-holding the camera.

One question: Were you using the waist level finder or a prism? I've only used the waist level hand-held...using the prism might be problematic because you'd be holding a pretty heavy camera up to your face. Just a thought...