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AI Servo tracking consistency is the best of any 1D series I've used, and it usually works well at 12fps too. Combine this improved shot to shot consistency with more reliable off-center focus points and the 20% higher fps rate than the 1DIV, and over the fall while covering football, I found I was averaging about 40-50% increase in technically acceptable keepers per event. Of course the creative value of many of these might be questionable, but the difference is these weren't immediately discarded due to problems such as flaky focus tracking. That said, it's not 100% perfect. I've had rare sequences where the subject went one way and the focus the other. I can't see how user error factored into those.
Buffer depth, particularly an issue in the past with high fps RAW capture, is finally pretty good, especially with a fast card. I would still like to see about a 30-50% capacity increase though. Side note: dual CF card slots has been a welcome addition, in particular when covering events in poor weather. With high capacity cards, it's possible to cover entire events without having to crack open the camera (other than maybe swap the battery). When prices come down a bit more, I could see myself working with dual 128GB mirrored cards for redundancy (though will have to test the impact this will have on buffer clearing and card write speed for some events).
High ISO is improved to the point where I don't think twice about 3200 or 6400. I recently shot a corporate event almost entirely at ISO 12800. At first I was a bit nervous about it, but with a touch of NR in post, you'd never know. One nitpick though: I'd like to see better high ISO (6400+) color quality.
Usable 61-point AF array subject tracking. I think this is the first iteration that has actually worked pretty well. The color tracking too, seems to be pretty decent. While it hasn't worked in all cases for me, it has eliminated the need to try to manually select the correct AF point for moving subjects in many situations where their movement is fairly unpredictable. I also like the camera's button customizability, which allows me to assign three different AF point array/pattern options - one each to the AF-ON, * and front function buttons. Great for on-the-fly changes.
Not as significant but still notable:
Better, more consistent auto WB.
Seemingly better and more consistent TTL flash performance.
I love the sound of the camera - it inspires confidence in its build quality. But, I do wish the 'silent' advance mode was quieter.
I'm very curious to see what the 7DII delivers. If AF performance is on-par with the 1DX (or within 85-90%), it could be an extremely viable alternative to older 1D series cameras. It will be interesting to see how much Canon tries to segment it and somewhat protect the 1DX. I'm sure this will occur to some extent, but if it's along the lines of what the 5DIII is to the 1DX, there might be some hope.