I am just starting through images I shot at an event over this weekend. I decided to let my 6Ds (I used two at this event) pick ISO, which I don't usually do (I haven't found auto ISO very reliable in past models). I typically shot manual everything, although on some occasions I will use AV mode if appropriate. I was using spot metering because the lighting was very dramatic. First of all, as long as I did my part (sometimes needs to use * to meter before recomposing), the camera was pretty much dead on with ISO. My images need next to no editing. This image was actually an exception: I had recomposed without first setting metering. As a result, this shot was overexposed. I dropped exposure 1/2 stop in post and pulled highlights down about -50. The only reason I point this out is to say that this shot wasn't perfectly exposed (AKA, not optimal conditions). I was shocked when I checked ISO on this shot because it still looked so good at 100%. This is ISO 25,600!!
Check out the full shot + 100% crop. This has had no additional noise reduction (my standard import from RAW is 10 Luminance and 25 Color in LR noise reduction). That has no editing other that what was described and a standard one click B&W conversion in LR on the full size image - the crop is the original color. Once again the clean output from the 6D has blown my mind - this is perfectly usable!