Some user info via http://www.northlight-images.co.uk/Canon_1DS_MkIII.html
"The firmware is still being tweaked, and testers get regular visits from a Canon rep to install new firmware and take detailed reports on problems/issues/performance. Some of the comments I've received (in no particular order)
Liveview focussing works a treat - people doing product work are loving it
Good lenses are showing their benefits, but others are not as bad as some had expected
The highlight tone priority is ideal for anyone working outside on sunny days :-)
People are using DPP to process the RAW files for the time being, although many are converting to DNG for further work.
Some of the amount of extra dynamic range that can be pulled out of the 14 bit files still needs some software tricks, but expect some pretty interesting results when the various specialist raw processing software vendors get to work.
Overall colour accuracy is a definite improvement over the 1Ds2, particularly noticeable in reds/skintones
The amount of fine detail visible suggests that a relatively weak (compared to 1Ds2) AA filter is in use. The improvement looks more than would be expected from the 16->21 MP increase.
The improved viewfinder really shows if you have to go back to using a 1Ds2
At A3 sizes(and above) prints and large glossy magazine images look noticeably better than the MK2
High ISO noise performance is much better (than the Mk2) with less chroma noise and better detail.
At 100 ISO there has been the suggestion of a bit more shadow noise - not visible in real work, and perhaps more suited to 'Angels on a head of a pin' arguments on DPR ;-)
Unless you are working in a very well lit studio environment the P25 MF back is easily matched, and a P45 can be bested if the lighting is not optimal."