After deciding that I could not justify the cost of the 1DX, I felt bad about stepping down from a 1D series camera to the 5DIII. My 1DII is on its 3rd shutter box, and has about as many activations on it as it did on each of the first two shutters. About to leave on a 3 week trip I decided to go for the 5DIII as a second camera body. I had also considered selling all my Canon lenses and switching to the Nikon D800, but the increase in lens prices made that a dismal choice, financially.
Well, after less than 3 weeks of playing with the 5DIII to get used to it, I am tickled silly. I shoot nature, cityscapes and occasional landscapes and birds, with a full range of lenses from 15mm fisheye though 500mm f/4. My impressions of the 5DIII is that with the exception of 3 things: the less-rugged body, the inability to autofocus at f/8 (500mm lens with a doubler), and the maximum frame rate of 6fps, the 5DIII blows away everything else about my 1DII.
The AF is faster and more consistently accurate, and the auto exposure seems to be at least as good. For somebody who likes to shoot inside churches and subways, the high ISO choices with much less noise makes me feel like a child in a candy store. And when I crop down a shot with the 5DIII to the size that I would have had with the 1.3-crop with the 1DII, it has more pixels and with less noise than the 1DII!
And with the Lexar 1000X 32Gb CF card in the camera, the ability to shoot bursts of over 25 RAWs at 6fps satisfies me for when I need to hand-hold the camera for HDR's. The in-camera shooting of multiple exposures is inspiring me to try those for creative effects that I haven't done since my pre-digital days (or is it "daze"?).
Add to that the ability to bracket from 2 to 7 shots in a bracket, which I often do for HDR's, and I now consider my 1DII as my second body.
I know many do not see the 5DIII as that much different from the 5DII, and it could be that my impressions coming from the 1DII rather than the 1DIV make the 5DIII look a lot more advanced than it really is, but I am one happy camper. And, as far as the ruggedness of the 1D series, my being on the 3rd shutter is because the first one self destructed at about 65K activations. I got about that on the 2nd shutter before it placed a metal strip across the frame. And the 3rd shutter has about 60K activations on it now. I'll be happy to use it as a backup for my 5DIII.
Oh, and after shooting all afternoon handholding the 5DIII with the Canon 100-400mm lens, I was shocked to find that without the extra pound or so of the 1dII, there was no ache or incipient cramping in my right hand! I had been worried that without the added vertical grip the 5DIII would not be as well balanced but for my very average-sized hand it turned out to be fine, and very nicely balanced and very noticeably lighter than the 1DII.