many people seem to hate the 6D AF system but it is actually very reliable at least the center point is.
"Hate" is probably the wrong word, it's about knowing the limitations, try to work around them (or not) and make an educated decision if spending nearly double the amount of money for 5d3 is "worth it" because of the af system.
the center AF point of it is more sensitive than any AF point of the 1DX , the 5D3 or my D800E.
I tested my D800E vs my 6D vs my D600 vs rented 5d3 in lowlight , the 6D actually handled extreme lowlight best, no contest here.
These tests are always interesting, but only if you can give the LV of the scene (iso, shutter, aperture, lens used), i.e. when the 6d starts to outperform the other cameras. That's because the 5d3 is supposed to af up to -2LV which already should cover most situations, so the question is how often you encounter -3LV and when the 6d af starts getting faster even above -2LV than the competition...
I was able to get mine to AF in light so dark that I couldn't really see what I was aiming at through the viewfinder, with my 120-400mm lens...at 400mm at f/5.6...at ISO 102k. (I had a little trouble with just my naked eye, but the viewfinder is a lot darker than the naked eye). The amount of light was, I don't know...there was no moon...it was 40 minutes or so after sunset. It was very late "blue hour", and I would say was about equal to midnight with a quarter moon showing, and clear skies. The target AF subject was the white belly fur of some deer...which was obviously facing the dark ground...but that fur was all I could see with the naked eye.
Needless to say the images sucked but it focused ok (took about a second to achieve lock). I shot them as lower resolution jpegs, and the noise is still not what I would call clean, haha. To make matters worse...in such dark conditions, you have to compensate exposure by +1 1/3, and even then it's still underexposed...at a slow shutterspeed with "OS" switched on.
Certainly that center point can focus in a lot darker light than is practical to take pictures that are worth looking at, even as an experiment.
But...what does this say about future cameras with future AF points? Seems like things will continue to improve. We all need to be saving for that 2014 1 series, don't we?