I’m still waiting for the Canon EOS 1D Mark III that I have on order. While it’s leaps and bounds ahead of the Canon EOS 5D in many ways, image quality will be the first thing on the mind of many people.
When corner and edge performance are taken into account, especially with lenses like the EF 24-105mm f/4L and the EF 16-35 f/2.8L, the 1D Mark III might look very appealing, keeping in mind that the latter two lenses will have the field of view of a 31-136mm and 21-45mm lens respectively (as compared with a full-frame sensor). The 21-45mm range is particularly appealing, so I expect the 16-35mm will see much more use than it formerly has.
What do I mean by “edge and corner performance”? On full-frame (36 X 24mm sensor), high corner sharpness requires stopping down to f/8 or f/11 with most lenses, and indeed with some lenses the corners remain less than sharp even when stopped down. On the EOS 1D Mark III, the sensor is 28.1 X 18.7mm (“APS-H”), which yields an image circle of 33.7mm, as compared with an image circle of 43.2mm for a 36 X 24mm (full frame) sensor. That cropped view lops off the poorest-performing portions of the imaging circle, meaning that one can obtain very crisp images even wide open.
For example, examining the Canon-supplied MTF charts for the tele end and the wide end of the EF 16-35mm f/2.8L II show that an image circle of 33.7mm (distance from center of 16.8mm) matches almost exactly the distance at which MTF (contrast) of fine details begins to plummet at the wide end. The situation is very similar for the EF 24-105 f/4L, the EF 17-40 f/4L, the EF 70-200 f/4L, the 24mm f/1.4L, the 85mm f/1.8, etc.
The tradeoff between sensor size and lens performance in the corners has been explored in great detail in D2x vs EOS. The 1D Mark III has a larger sensor and larger pixels than the D2x, so I expect it might well offer the ideal compromise between full frame and cropped frame (1.3X field-of-view crop).