Mike Fulton: With my 1DS Mk3, I shoot mainly at ISO 100 with studio flash.
The increased frame rate of the 1D X does nothing for me, because my lights don't recycle that fast. The improved ISO sensitivity does nothing for me, because I shoot at ISO 100. Wasn't having any focusing issues, so the improved autofocus isn't something I really need. Going from 21.7mp to 18mp is a drop of nearly 20%. Doesn't seem like a good thing to me.
The new camera sounds like an awesome step up for 1D Mk4 owners, but tell me again how this is an "upgrade" from my 1DS Mk3?
I think I'll stick with my 1DS Mk3 for now.
A slight decrease in resolution isn't good, but the size and pixel density is. I went from the Canon T2i to the 1D Mark II; in most aspects of that T2i, it beat the Mark II out of the water, 18mp, newer process (Digic 4), A/D Conversion was a tad bit faster 12/14 bit...These are all things to consider, look even what I gained; 8.5fps, all pro exclusive features. But what really got me to the Mark II was the way it handled with highlights and shadows; AND the slightly larger sensor and larger pixel density, (makes for a better high ISO/Low noise ratio. I honestly knew too much about cameras and related tech to own an entry level SLR, so I made the purchase on the Mark II; no dust on the sensor, no Error 99, no hardware flaws, crisp 8mp images.... What I'm trying to say, is that 21mp full frame might not be just as good as you think it may be if you put forward the effort to process the technology in such a matter. Look what Nikon did with the D3s, leading in Low light photos at 12mp!
I really look forward in getting this camera; upgrading from the 1D Mark II.