I'm using a Pentax 67II, and I think this mirror slap thing is totally overblown. Perhaps the earlier 6x7 and 67 are worse, but I'm getting just as sharp results with the 67II at 1/30 second as I do with my Nikon D700. The inertia of the camera is large enough to actually dampen the huge mirror movement. I'm not familiar with longer lenses as I only own the 75/4.5, 90/2.8 and 105/2.4.
For me, the P67II is a camera
for handholding, and a very capable one. The grip is awesome, the viewfinder bright and nice, the metering accurate.
Now, since I don't do prints (yes, I know, I'm mad), my Epson V700 gives me more than I'll ever need. But, after some testing I've found out that it can deliver a resolution that is higher than what the D700 gives even with quite grainy 400 B&W film, developed at home. A better scanner with slower film will certainly blow away your D3S in terms of resolution.
But the most important thing to me is the look of the images, that I can't get with digital. Also, I love the narrower aspect ratio that makes me feel "at home".
Here's the legendary 105/2.4 wide open. It's like using a ~50/1.2 on 35 mm, but SHARP and with no spherical aberration even wide open.
Edit: For some stupid reason the images are resized, so here's the larger one I wanted to post:
http://img.photobucket.com/albums/v621/Makten/Pentax%2067II/img005-2.jpgOh, and handholding? This one is with the 105 @ f/4 and 1/30 second, handheld...
http://img.photobucket.com/albums/v621/Makten/Pentax%2067II/img004-3.jpgYes, there is traces of camera shake if you examine at 100%. But not worse than I would have had with a 50 mm lens on the D700.
And finally, here's a quick test regarding resolution:
http://www.pentaxforums.com/forums/pentax-medium-format-645-6x7-645d/134572-nikon-d700-vs-pentax-67ii-pictures.html