Several people have found it very beneficial to have two buttons set to back button AF. I have the AF-ON button set to Eye-AF. But then the * button is spot AF. Depending on my mode, I have the trigger set to expanded AF, but several modes are back button AF only. What is nice is that if you are holding one of the back buttons, that mode will override the trigger.
In short, the AF system still follows algorithms that seem to favor closer subjects. That is for sure, but I feel that it also favors high contrast/brighter subjects. But, bottom line, in full auto mode it will grab onto some subjects that are not your intended target. In these instances I either quickly switch to spot AF (quick movement of my thumb from one button to the next) or I train the camera as to what I want it to focus on using Spot AF, then I switch back to Eye-AF to nail the eye.
As to how to program the buttons, two methods. I tend to toggle the info button until the Q mode settings are displayed, then hit the Q button and select the back of camera icon. This brings you to the customize button menu, scroll down and set AF-On to Eye-AF, and then * is set to AF (but when making this selection, you can hit the info button that that lets you select spot AF).
So, that is how I do it. I have seen several videos where others have similar set ups (Jan Wegener, Whistling Wings Photography, etc).
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