I will likely rent one sometime and give 4k frame grab, 4k vid, and the diffraction correction (I am certainly thinking John is in the right direction on sharpening, but time will tell), as well as, testing the 35-50 1.8 as a vid lens (rental too).
I am certainly not all giddy about running out and upgrading - if I get GAS at all it is toward a 1dx1. I am much more likely to just shoot the heck out of what I have and wait until there is some sort of serious wear out and then see what is about.
The improved feature set is "handy" but I am quite pleased w/ the images the 5d3 kicks out. Perhaps when the 3d, physics defying, 18 stops of DR, completely ISO invariant, 30 FPS full res, global shutter 8k at 120 FPS vid, everything and the kitchen sink model comes out....