If you're going to use the center AF point only...

The 6D outer points are accurate and usable. Perhaps not as fast as the 5diii, but they seem to reliably find their intended target.

Personally i use all. If i recompose i will use the closest point or rotate the camera to avoid moving the focal plane much. For still targets and landscapes where one might want to target something at the sides, live view is an option. Face detection works very well.

When not tracking i use center point when there is virtually no light.

When tracking i tend to use all focus points, thus starting at center. Sometimes i might use center only for tracking, but that is not too often.

With the 6D i can more often use all focus points compared to the 30D, since the frame is bigger. I say this because when using crop sensors with the same AF point pattern, you will more often have to rely on one focus point alone to avoid obstacles in the foreground (assuming the same framing). For the 5diii, there are of course more options available making it better.

I would probably use the outer focus points alone more if there were a good joystick such as the 5dii have (the 30D joystick was not as precise as the 5dii). But still i prefer the 6D outer focus points to the 5dii, which is less reliable in my experience. I use the multi-dial to select focus points as i would with a joystick, but that operation is slightly slower.

Before i bought the 6D i was disappionted of the diamond pattern as well. But i realised that compared to a crop camera, i can crop my way to the closest points from one or two sides, still utilizing larger sensor area than the crop camera would allow me to have. I also realised that the reduced weight of the 6D compared to a 5Diii would be benificial to me (ie sometimes the 200g difference at some point would mean the difference between bringing or not bringing a camera.) I'm not saying i would not love having the full 5diii AF point layout and speed, but it was not cruicial to me, and thus the price difference would be better spent elsewhere.. .

-Yngve