So I would appreciate feedback regarding the quality of the autofocus from people who use the 5D III or 1DX. Critical, honest and heavy users and people who have to deliver to customers. Is the autofocus in the mentioned cameras good - ore just LESS BAD but still not as it should be with such a price tag? Your feedback would help me a lot!
I shoot professionally using a variety of Canon cameras, including the 7D, 1D3, 1Ds3, and 1DX. I am more than a little familiar with traveling several thousand miles to do a photoshoot which can not be shot a second time.
The 1D3 and 1Ds3 were good bodies for me until it came to a shallow DOF. I could not rely on them to be tack sharp where I needed, and it was not a matter of front or back focusing. However, in my case it was a matter of inches at worst, never of several feet as you describe. Thankfully much of my work is at higher Fstops, so the issues were typically not of concern.
When I received my 1DX, it was like I had found a new style of photography. I must have been simply used to working with the older AF systems, because the 1DX locks on in low light, and hits with ALL lenses at ALL apertures. My glass suddenly seems new, and I am more than a little impressed with how well the AF works.
I could be bitter at the AF of previous generation bodies, but it worked well enough for what I needed. The newer AF system is simply stunning. While I didn't enjoy the price of the 1DX, it is all part of business, and at the end of the day it is making me money and allowing me to shoot in conditions I would not have been able to previously.
I debated heavily getting rid of my other bodies and picking up a 5D3 to go with the 1Dx. I'm keeping my 1Ds3 for studio work and product photography, and the 1D3 is being kept because there is little value in it. The 7D is a "beater" camera, and is taken up into or under the mountains and in other conditions where I need a light camera and top notch images are less important (though still able to be sold). Being able to keep a supply of similar batteries on hand is another big plus.
I've never owned or used a 1D4, so I can't help you with a comparison, but the 1Dx does everything Canon claims in my opinion.