I've tried it, and I've read the manual, page 271 and page 277. I can't really tell if this method is helping much. Maybe I'm expecting too much? I try it with my son, but he is only 3 yrs old, so maybe his eyes are too small? I am in close.
What happens is the AF square shrinks to fit the eye and then, as long as his face is not jumping too much, and not turned towards me (instead, partial profile), the eye AF works ok. But if he moves, or I move, the square expands and shrinks, sometimes jumps to the other eye or even the ear nearest the camera if he turns so the opening is visible.
I'm just not seeing how eye AF really works better than just using Spot AF and keeping the AF square on the desired eye. Maybe if the posing is kind of slow and predictable it keeps one eye in focus or jumps to the eye closest to the lens...
I'm still not really sold on this feature. But thank you for sharing Canon's preferred way of helping it work better!