In addition to the ISO 12233 charts and the color patches/fabric shots in the growing 1D X review, maybe folks would like to see some real world images...
This was a small outdoor jazz concert at a local science museum, featuring Nathan and the Zydeco Cha-Chas. The shots below were taken about 30 minutes after sunset, ISO 4000 and 12800.
Shot RAW, converted using default settings in DPP, only edit was cropping. Click image then View All Sizes (upper right) for a 1600 pixel version. Thanks for looking!
EOS 1D X, EF 70-200mm f/2.8L IS II USM @ 200mm, 1/320 s, f/2.8, ISO 4000
EOS 1D X, EF 70-200mm f/2.8L IS II USM @ 200mm, 1/250 s, f/2.8, ISO 4000
EOS 1D X, EF 70-200mm f/2.8L IS II USM @ 70mm, 1/200 s, f/5, ISO 12800
The 3rd shot should have had an even narrower aperture for a little more DoF, I think. That's my older daughter on stage, singing into the mic.
One more, from before the start of the concert, my younger daughter at the bubbles exhibit. A 12 fps burst is great for capturing a key moment.
EOS 1D X, EF 70-200mm f/2.8L IS II USM @ 70mm, 1/200 s, f/2.8, ISO 100