I received my Mark III a few weeks after they started being delivered. I have a bazzillion clicks on this thing, but it keeps on clicking. I've never had the focus issues other have reported, in face, my Mark III always focuses great. The Microfocus Adjustment makes the AF even better.
Recently I was shooting with a 200 f/1.8 + 2.0x + 1.4x. The AF still worked on all AF points, although it hunted a bit on low contrast subjects. The results were outstanding. The focus was spot on even with all that glass in the way. Can't say enought good about the Mark III.