A 100% crop of a mountain bike sample photo taken at dusk in the woods with the Canon EOS 5D Mk III at ISO 6400, using AI Servo continuous auto focus (original image is here). I think this is a really impressive photo. Yes, you can see a bit of noise. But I consider this perfectly publishable quality for an image shot in these conditions. Add the continuous auto focus into the equation and this is a photo that arguably would have been impossible before the Canon EOS 5D Mk III.