After a few weeks of shooting with the ZD back these are my thoughts:
The back is FAR from perfect but for $7k it's the best bang for the buck in MFD. The LCD sucks and would be completely useless if not for the histogram. The previews take forever to appear and on a modern digital back that's unacceptable.
The buffer can be problematic if you're not paying attention. I recently photographed Dick Vitale and he's an animated guy. After a short period I hit the buffer and had to stop shooting. He's not the kind of guy that wants to stand still and so it was a little uncomfortable. For fast shooting you're better off with a DSLR or renting a Leaf 54s or A75.
A minimal amount of uniform noise will show up in fabrics at ISO 100 if noise reduction is turned completely off in your raw converter of choice. Just a little NR does the trick.
The files overall are crisp and the color is is dead on. It's always better with the ZD to shoot a gray card and not depend on the AWB.
The ZD back can handle long exposures with some color NR. I've gone to 10 sec's and gotten great results.
Now, would I have bought a ZD back again knowing what I know now - maybe. The first ZD back I received was defective. Purple worms, bad color noise at ISO 100 and shoot interrupting lockups. Mamiya was VERY responsive and replaced my back with a new one and things have been great ever since. With the price of used and refurb Aptus 22's dropping to around 10k I would consider one of those first before the ZD back. The larger buffer, fast previews and LC 11 software is worth the extra $3k IMO.
Unfortunately, the games that the back dealers like to play with prices and value-add warranties, etc. is just dumb. And here's where I'm going to probably piss a few people off - the process of buying a digital medium format back is akin to buying a car in my experience and we all know how much fun that is. How a refurb Aptus 22 goes from $16k to $13.5k in less than a month is beyond me. How the same back with warranty can be offered from another dealer for $10.5 is insane. Don't tell me about warranties and support - if I'm paying $7k or $10k or $25k I demand that the back work right out of the box with no if's, and's or but's! I shouldn't have to have my MFDB dealer on speed dial for support when I've paid $20k or even $7k. It's amazing that I can buy a Canon 40D for $1200 and it works and I don't need to call my rep at B+H to get the inside scoop on how to set the camera up.
Bottom line - I wish I had taken James Russell's advice 2 years ago and just stuck with my Canons. I've shot enough with the ZD back, Valeo 22, H3-39, Canon 1Ds MKI + MKII, 5D and even the new Nikon D3 to believe that the perceived quality of MFDB over DSLR's is more perception than reality. Factor in the difference in price and it just gets stupid.
Sorry for the rant at the end of this. I'm going to take the advice of Mark Tucker and shoot more pictures rather than shoot my mouth off any more.