I do not claim to be an expert but I have used a number of DSLR cameras over the years and have to say that the 1DX is the camera that has made the biggest difference to my photography. This is because the 1DX allows me to (almost) forget about the camera and concentrate on my shooting! My primary photographic interest is wildlife/bird photography so please bear that in mind.
1. The downsides. The 1DX is hideously expensive, even the prices of grey imports are still pretty scary.
2. - it is BIG. The physical size of the camera can be a bit inappropriate/intimidating i some situations - not for wildlife as they don't care! Though I did feel a bit of a twit at my Grand Niece's 1st birthday party the other day!
3. - weight. It's heavy! I came to this camera from the 1D Mk4 and was surprised at how much heavier it feels in the hand. I believe the difference is only about 200 grams but it can be significant.
4. - Menu. Though the new Menu system is easy and quite intuitive to navigate it is slow as you need to go through every page in between to get to the one you want - perhaps I have missed something here?
5. - the Multi Function button - I don't have hands like shovels so I can't reach it easily!
6. - AF On button in portrait mode/orientation. In Landscape mode/orientation the AF On button naturally falls under my thumb (I use Back Button Focus) however in portrait orientation I have to search for it - this has cost me a potentially very nice reflection shot of a Bittern.
7. - Battery life. Still pretty good but the 1DX chews up batteries quicker than a 1D4 and much quicker than a 1D3 - so budget for an extra battery or two. Mine doesn't report battery life with non OEM batteries and they don't last as long but at 1/10th the price who cares?
1. -AF. Up until recently I have been using inappropriate AF modes/cases. The result of this was the best AF I have yet used. Now I have changed to case 5 it is even better! My, previous, Canon 1D4 was excellent - the 1DX is significantly better/quicker under all conditions that I have shot in to date.
2. -Metering. The 1DX allows a fair amount of latitude for recovering incorrect exposures - or so I am told. I cannot say if this is true as mine has, yet, to get the exposure significantly wrong. Yes there have been incorrect exposures but they have been my fault not the camera's.
3. - Auto ISO. Lots of cameras have it but with the usable ISO range of the 1DX it really comes into it's own. The default range is up to 12800 ISO which I feel is sensible as above that you may need NR. With my previous cameras usable ISO range was much more restricted so I preferred to set it myself.
4. - EV Compensation on the fly. With the latest Firmware you can adjust exposure compensation whilst in Manual mode and using Auto ISO. I have mine set so that I press the SET button then adjust the exposure compensation with the MAIN DIAL. I find this so quick and easy that I can use 3 or 4 different exposure levels on a passing Aircraft at an Air Display - very handy as the bank into and out of the sun.
5. - Speed. This camera is fast, seriously fast! I am not thinking of FPS as I have reduced mine to 10fps. What I mean is that it responds faster in all departments. There is no, noticeable, shutter lag, AF locks and tracks better/faster than anything I have yet tried from various manufacturers. With a fast card (I use Lexar 1000x 32GB) it dumps RAW files very quickly. At 10fps I get 51+ RAW files before it slows and the buffer is fully cleared in a couple or three seconds. That allows 5 seconds of RAW files at 10fps - better than any Canon yet made.
6. - The big one in my opinion! When this camera is set up to your liking it is VERY simple/intuitive to use, far more so than a point and shoot in P mode! Generally I leave it in Servo AF (I just don't press the AF On button when I want MF) and it doesn't hunt back and forth like previous models, Auto ISO - because it has such a wide usable range. Proper AF point selection helps but, so far, some of my best BIF shots have been with single point Spot AF (no expansion AF points) which is totally inappropriate - but it works even you do drift off the subject - very handy when you are caught out!
7. - IQ. The files that the 1DX produces require little, if any, processing under most conditions. I hate mucking about with images so I like cameras that give me what I want. For competition entries I will use a touch of sharpening and lift the image a little if the light is poor - but not much. I am not saying that the 1DX makes Photoshop redundant but, if the light is good, files need little work and not much more in poor light.
I hope that lot makes sense!
Overall I am VERY pleased with the 1DX, it is not perfect, but it is the closest thing to perfection currently available for my uses. It is not the camera for everyone but for birding/wildlife photography it is the best I have found. Many make a big deal of crop factors but personally I have found that, whilst I do loose some reach, it is nowhere near as much as the 1.6 crop factor would suggest. More importantly I am still getting good images at F8 when APSC users, with 1 or 2 stop faster lenses, are going home.
As you may have gathered I like this camera!
Any comments/queries are welcome, as I have probably missed quite a lot!