So, I have a 6D and a D600 in hand right now...

Amen to that ! I dislike top left controls anyway.

The previous incarnation (D700) was utterly annoying, but I agree that switching from a hard button to a soft one has improved a lot the situation. However, as with every controls activated by the left hand, the efficiency of these controls heavily depends on the lens in use. As an example, I find the new 6D's DOF button quite good when operated with normal-size prime lenses, but unreachable with long lenses or very awkward with pancake lenses. Same applies to the AF switch on Nikon cameras. I'm not a huge fan of Canon's version either, for other reasons obviously.