I'll keep it very short, because I could ramble on about this Neo Anderson of cameras for days.
I have neverever even for one second regretted the upgrade. If one thought the mk4 was a big upgrade over the 1d3, you'd be blown away over this upgrade.
Why I love it: Fullframe with superior fps AND the best af ever, not just 90% good, but in many cases 100% hitrate.
The layout, the vf, the grip and feel. But most of all the metering and general image quality. Noise is almost not visible at normal size print at any value under 25600 iso.
What I don't like is poor battery, keep at least two! The new iso button.., too small and weirdly placed. And from what I have found, or not found, the ability to spread the bracketed images on the ev scale with the small wheel and move the whole scale with the rear wheel, like I could on the mk4.
Very small minuslist and about battery, if you hold the button down in a soccermatch I've seen people get 5000 images on a battery but I get around 1100 images, and compare that to the mk3 where I got 3000 easily and you see my point.
1d x THE perfect camera for me, you get so many fantastic moments you just don't get with anything else. Yes one can time the perfect moment and not "need" 12 fps, but not if the AF can't keep up, and with 12 fps you have 12 perfect moments you have never seen in just one second, pretty frikkin' awesome..