Playing with the 5D Mk III on the weekend, it's most outstanding performance was when taking landscape and nature photographs, more so than astronomy I felt.
The sharpness and ease of focus was as I remember with my 35mm film. The 7D was a great leap in focus capability and when I first switched to it I felt I had arrived back at what 35mm film gave me. However the 7D's small pixel size makes manual focus and sharpness a challenge at most focal ratios, for me anyway - not so with the 5D Mk III.
http://www.rogergroom.com/items/purple_swamp_hen
The crop (main thumbnail image - click it for larger) is 100%. The link on the page shows a resized image of what the full crop was.
The photo was shot at 1600ISO.
Roger.