i looked @ ProBack as well. Kodak will cease to even support these backs Dec. 31 2007, so anything that goes wrong, you may be up a creek. once I discovered this, I completely ruled out the ProBack and will continue w/ the 5D for a while, renting MFDB when needed.
There are pelnty of other discontinued MFDB around that you can still get service for (namely Phase, Leaf, Imacon) so why risk it? These other back may be slightly more money, but think about
upgrade paths as well - no such path for the ProBack.
also, I've seen RAW files from different ProBacks w/ tons of hot pixels and "lines" down the frames - these were in normal exposed images, not long exposures or anything. One of these backs had recently been serviced and reconditioned by Kodak.
The one spot that the ProBack excels, the ablity to shoot untethered. most system sof this vintage
required you to be tethered.
IMHO, I would tread wisely forward into the ProBack abyss...
jim