Beware, there is no TC that works on all Canon lenses. The Kenko DGX with blue dot was the latest I owned, and it locked up my 5D MK III so that I had to remove the battery to reset it when used with my 100mmL macro lens. Later, I found that it locked up with a few other Canon lenses as well. There are lots of threads about the issue on various forums.
I think it works well with the 7D, google to find information about the 6D.
The bottom line is that Kenko keeps making firmware fixes to this TC. Once you buy it, you are locked in, and it might not work with new lenses on existing bodies, or on new bodies at all (read 7D MK II).
It seems that new AF systems from Canon are the issue, they work differently, and the TC is reverse engineered for older systems. We certainly are hoping for new and better AF systems.