I recently traded in my Mamiya 6 for a Pentax 67ii with AE prism and 90mm lens and am pretty happy. The 90mm lens is compact and light, and has a lovely signature. Of course, compact and light doesn't mean much when the body and prism weigh so much. Everything feels wonderful, from the solid body to the smoothness of the focusing. Close focus is 0.65m, which I use a lot. The viewfinder is big and bright, and there are no problems acquiring focus. I'm growing to like 6x7 format too.
I shoot handheld only, and have achieved satisfactory sharpness (for me) on at least one shot at 1/25 s, so the mirror slap issue has been a red herring. What has been more of a problem has been the weight. It's heavy to carry around. Once raised to the face, it handles brilliantly, but I have to say that when shooting in portrait orientation, I think my hands are less steady than they might be, so I tend to opt for landscape orientation.
I'm verging on ecstatic. This is a wonderful camera. The only other criticism is the max 1/30 s flash sync with most lenses, but I don't use flash.