Thank you so much for the feedback, EWP. As someone in the market for a good birding camera, I've been considering the merits of both the 5D III and 1D X. I really don't want to spend the kind of money I would have to on the 1D X, as I am primarily a hobbyist, with a sale of a print every great once in a while. Your opinion on the 1D X shutter sound might be sealing the deal for the 5D III...I think a quiet shutter would be very helpful when it comes to shooting jittery birds up close (even within a blind).
I did want to comment on one thing, though. Regarding ISO:
East Wind Photography said:
• Shooting up to ISO 1000 I did not see any improvement in noise levels. I’m sure it’s better but not enough to notice in real world situations.
This is not really surprising. The improvements in ISO are really only going to be apparent at higher ISO settings. While the improvement in full well capacity for the 1D X, thanks to the larger pixels and higher Q.E. (47%) over past-generation sensors (which were closer to 25-38%), will result in the same absolute improvement at all ISO settings, however the relative improvement will increase as ISO is increased, thanks to the nature of photon shot noise. At lower ISO settings, a well-exposed (ETTRed) photo will have roughly the same absolute differences in pixel saturation for a given tone as at higher ISO settings, however because the maximum saturation is lower by orders of magnitude at say ISO 12800 than at ISO 100, those same absolute differences become much larger relative differences, and appear to result in far more noise.
At ISO 3200, for example, the 1D X has a three-fold advantage over my 7D. The 7D is quite noisy at ISO 3200, but the 1D X is incredibly clear. From the bird photography I've seen that was taken with the 1D X, photos taken at ISO 16000 to 25600 appear to be about as noisy as ISO 3200 on my 7D. That is nearly a three-fold improvement in high ISO performance (and probably more so, given the much better editing and noise removal latitude that the 1D series has historically offered over lesser models.) I would be willing to bet that you could push ISO to 6400 or more in your work, and in comparison to older cameras you have used the 1D X would fare increasingly better as ISO was increased.