I shoot a lot of sports. Hence, any improvement I can make to my tools or skill set that allows me to get more keepers at events is a good thing. I'm fortunate to be able to pick and choose my gear at this time, so I've tried to find the best gear for the money and work to become better at my craft. It wasn't until about 4 years ago that I really started using the full feature set of my camera - i.e., I got serious about making the best use of what I had. Buying a 7D was an eye-opener: here was a camera where the limitations were all mine and not the gear. It was a big step up from the Rebels I'd been using, though not really in image quality; it was more along the lines of usability. The camera was comfortable, big enough to grip in a stable manner, and configurable to meet my needs.
I used the 7D long enough to grow as a photographer and find its limits; I didn't realize how much I had yet to learn when I started. The few things which I felt were now holding me back were addressed by the mk II, so I upgraded. Sort of in parallel to that, I wanted a FF camera both for flexibility and to match my existing lenses better (zoom range can be critical to getting the shots in some sports, and work far better for me than primes; the 70-200 is a much better fit to the FF than the APS-C camera, while if there is enough light the 24-105 is a great fit to the 7D2) so I added a 5D3 that I got for a very good price. I use both about equally, with the 5D3 in use more often for portraits and such whereas the 7D2 fits sports events a little better most of the time.
But it became apparent that even after lots of work choosing AF parameters and revising my shooting style, the 5D3 just didn't have the AF capabilities of the 7D2. In sports where there are no re-do opportunities, a missed shot is gone forever. Spray-and-pray works sometimes if the camera can focus, and delivers a string of OOF shots if it can't. With strobes, that's generally not an option. Neither is a disaster, but I hate losing a well-composed shot just because the combination of circumstances rendered it out of focus. After a tournament where the 5D3 did a good job but still inexplicably missed shots which weren't my fault, I was willing to expend the effort to find out where the problem lay. The bottom line was that I found a demo 1Dx online (w/warranty) for a deeply discounted price (wish I'd had that option a couple of years ago!) and deemed it worthwhile. I got it, checked it out, and shot a few events which were challenging but not vital.
One thing I noticed immediately: when the 1Dx focused, it was dead accurate, very much like the 7D2. And the image quality was great with minimal tweaking; I've never felt that 18MP was inadequate, even back in the days of my T2i. (Side note: IQ with modern sensors is much more a function of the lens and the camera Raw importer than the camera per se. Sure, the 1Dx images look "better" than those from my 7D2 at a glance, but if you take the time to PP them both carefully, there's not that much difference. No one's Raw importer - not even Canon's - processes all camera images to the same standard and the eye can be easily misled. I once took shots with my 7D and my 5D3 at the same resolution to prove this; with careful PP it was very hard to tell which was which. But I digress.) The result, though, was not much better in terms of keepers than the 5D3. Why? I did even more investigation of AF cases and my style and came to the conclusion that the combination of this camera, my shooting style and sport was very sensitive to the accel/decel parameter. I already knew that I needed minimum tracking sensitivity for best results, but apparently the camera responded too quickly to small changes in speed of the subject, and focusing became unstable unless I backed down the accel/decel value (as it suggests in Canon's manual).
After changing this one parameter slightly, I got nearly 100% keepers at the next 2 events (over 500 shots total). Would this help on the 5D3 too? I am going to try it, of course (the settings menu is not quite identical, and it didn't help with the 7D2), but I don't think it will entirely solve the issue; part of the advantage of having a 1Dx is that it will drive the big USM lenses faster, which minimizes time spent OOF. This is noticeable in practice, and if I get the AF point on the subject and contrast is high enough, the 1Dx will cling to it a little better. Enough to make a difference, anyway. So what is the lesson? That AF behavior is the user plus the camera plus the settings. You can't solely blame the camera until you've shown that the other two variables have been eliminated; that can take a while, depending on your desire to understand the details. In no case have I ever dealt with a faulty camera - it's just that I'm really critical of focus and striving for that unattainable 100% is something I'm willing to do. All of these cameras are pro-level in terms of autofocus, really; if you're not OCD about it, they'd probably be almost indistinguishable.
So that solved my focus concerns; what else about the camera? I love the ergonomics of the big Canon bodies, and the shape that lets me wrap my hands around it securely; I despise the tiny angular-shaped bodies which force me to grip them with my fingertips and push the buttons at the same time. The 1Dx is a heavy beast, that's for sure, but if I can hold it with a 70-200 f/2.8 all day at 63, I think anyone who's in decent physical shape should have no more trouble with it than with the APS-C bodies. The controls are much the same as any other high-end Canon, which I'm totally fine with; but there is one thing which bothers me more than I thought it would. I know it's done for reasons of weather sealing and reliability, but I miss the big mode control dial. Trying to find the Mode button and cycle through the various settings is frustrating; I hold the camera from the bottom so I have to shift my grip and poke around for the button in between shots. What I've done is disable everything except the C1-C3 and M, which is a workaround but still takes longer than I want it to. And I do change modes during an event, so I'm reminded of the discontinuity every time I pick the camera up.
For some reason the power switch feels like it operates backward (up is off). Of course, I'm the same person who is frustrated by zooms that operate counter to the Canon direction; but it is noticeable, every time. I really hate having to look at the lever to be sure it's in the right position; every other Canon is flip to the right "on", flip to the left "off". I know they technically both rotate in the same direction, but it just feels wrong. These two items are all that keep me from rating the camera a 10 for ergonomics.
I like the added M.Fn controls on the front, but haven't gotten accustomed to using them yet. Seems like they're a good idea, especially when they can be programmed to do a number of useful things. I don't use the rear storage/info display at all, but maybe some people do. The big battery seems to last a lot longer than the ones on my gripped 7D2 and 5D3. It's about $100 too expensive, and there is no 3rd party replacement that fully communicates with the camera, so at the moment you're stuck - and probably it will always be so.
That's about it, really. In actual use, this is the best camera I've ever owned. Even my 7D2 felt a little poky after using it for a while, and that's not a word I would ever have used to describe the 7D2 before now.
Overall, I'm happy with the camera. I wouldn't have paid full list price for it - and I'm not a pro, so I couldn't write it off as a business expense. But for about the same as the original list price of the 5D3, I got a bargain. It will accompany the 7D2 to all my events for the foreseeable future.