With the Canon EOS 1D Mark III, Canon takes us another big step forward in digital SLR technology. Redesigned from the ground up, the 1D III has so many new and improved features that it is hard to know where to start. Actually, it is easier to start with the only feature that is not best in class - The 1D Mark III does not have the 16.6mp resolution full frame sensor of the Canon EOS 1Ds Mark II. Otherwise, the 1D III is better in practically all regards. 
 The 1D III makes my 1Ds II (from the 1D Mark II and 1D Mark II N era) feel very much like it is due for an update. I have been extremely happy with my 1Ds II - it is an awesome camera, so generating this feeling is not an easy accomplishment. But there are just so many new and improved features on the 1D III. Read the very nice review here