I've had a Mamiya 645 Pro TL for the last few years, and I have enjoyed using it as a handheld SLR. With the metered prism and the motor winder it really is a pretty good handheld camera, on par with my Nikon as far as feel. It is however louder than the Nikon. The motor makes the most noise, but it's not bad at all if you use the hand crank. But the motor grip makes the camera much easier to use handheld. There are two winders for the Pro TL, I use the smaller one and it hardly adds any weight.
I have had plenty of good shots at 1/30th or slower, as long as I brace my body. For me no SLR is as good as a rangefinder/TLR with slow shutter speeds (I must twitch as the finder goes dark).
I listed it for sale a month or two ago. There are some pictures of it here, but none comparing the camera to the Pentax.