I've have the DCS ProBack and believe the images are more detailed with better tonality than the 1Ds2 that I had. I'd also agree about the ISO, 100 is best. There is a fair amount of noise at greater ISO, particularly in the blue channel. (This could be reduced using LAB and b-channel smoothing.) I've made prints from the ProBack up to 24 x 24 inches on an Epson 7800 that are extremely detailed.
You can buy these backs for around 3-4k. However, also as stated, you cannot use the DCS ProBack off the dedicated body for which it was designed. Only the model with the moveable LCD can be triggered on a view camera (this is the "Plus" model). And this requires a special cable release. (See http://www.kapturegroup.com/proback/plus.html
for such cables and adapters.) There seems to be no way around this matter.
As for usage, I do find more lock-ups on the Mamiya 645AFDii with the ProBack vs the Leaf 65. The Kodak frequently locks up, requiring a power on and off; the Leaf, rarely (only when I turned it on and had no CF card inserted).
If you get a ProBack, make sure the IR or AA filter is in good condition since the replacement cost is $1200 from Kodak. The IR blocking filter is probably preferable as the moire from these backs is not so much, and the image is sharper with the IR as compared to the AA filter.
This removeable filter feature does offer the possibility of IR photography. I've tried it and it actually works well, with proper autofocusing without the filter on a Mamiya 645AFDii. The main problem is trying to fit an IR band pass filter in place of the stock IR blocking filter to avoid placing one over the lens.