Hello Greg:
Again with every camera system there are trade offs. I have both a RZ67 proIID and 645 AFDII with a Leaf Aptus 75S. The RZ is certainly not as easily a walk around camera and you do lose autofocus and fast shutter speeds since the RZ is limited to 1/400th. But you gain flash synch at all shutter speeds with the RZ, even up to 1/400th. With the RZ you will lose the ability for wide angle shooting because of the 1.5-1.6 crop factor with a 48 x 36mm sensor. But that is an advantage in the telephoto range giving much more apparent reach. My 350 APO RZ becomes a 525mm.
Now as to lens quality, Mamiya makes fine lenses for both systems. Based on my personal experience owning both systems and fairly extensive lens collections for each, the RZ lenses, for me, have a unique presentation more commonly associated with fine German optics. Now not everyone sees the differences, but I see a 3-dimensionality to images shot with an RZ lens compared to many of the 645 AFD lenses. Some attribute this 3-dimensional look to better rendering of micro-contrasts. I don't know about the technical explanations, but I see and enjoy the difference. Now I don't have the newer AFD D-series lenses like the 28mm and 150mm, so I can't comment if those have that look I like, but from some of the jpegs posted on this site taken with those lenses, I suspect they might.
Other advantages of the RZ over the 645 AFD include a RZ tilt/shift adapter with special short barrel lenses for t/s that will still give infinity focus. Again, no wide angle here with the RZ crop factor and a MFDB.
So, I do find the RZ lenses quite sharp, sometimes sharper than the AFD equivalent for 645. I know others on here have owned both systems, Jack for instance, and hopefully they will weigh in as well.
So is it worth the $1000 or so for an RZ/Mamiya digital adapter (I think only works with the pro IID), or $600-$700 for one of the others (Phase, Leaf, Sinar) and a synch/wake-up cable? For me it was worth it as I use each system based on what I am photographing. I do take the RZ out into the field for landscape if I am not needing wide angle....of course on a tripod. For more walk around, faster dslr-style shooting, I will always take the 645 AFD.
Jeff