A very well designed camera with streamlined shooting design, the Mark III makes some intelligent improvements upon the Mark II.
A bit bigger
It's a bit larger and heavier than the 5DM2, as well as the D800, but it doesn't fee too heavy and it's well balanced. It's a little more comfortable to grip than the 5DM2, thanks to a "stickier" texture. While the body isn't as tough as the 1D series, it has a little more dust-and-weather sealing than its predecessor.
Another welcome change from the 5DM2 is how much more physically configurable the camera is. In addition to this programmable function button, you can assign another function to the depth-of-field button (moved to the right side for operation with your right-hand ring finger).
Though I'm sure a lot of people will welcome the lockable mode dial, it's got the design I dislike, which debuted in the 60D with the pushbutton lock in the middle of the dial. I also don't like the new location of the power switch; I don't know whether it gets moved when I put the camera in my bag or take it out, but whenever I take it out it's set to "on."