I have something coming up where we're going to go crazy low light using 2x Mark II's along with 1.2 glass. I want to see how far I can push the ISO in the camera so I set up a little gray card / chip chart test to have an objective look at the midtones, shadows, and highlights as well as how basic hues are affected across the full range of ISO from 100-6400.
This is a full resolution 1080 video file so watch it full screen in HD to really see what's happening to the image at various ISO settings.
Despite compensating the stop on the camera 1/3 for each ISO bracket, there still is a slight amount of exposure difference in each setup.
My thoughts: ISO 100-320 are extremely clean. 400-800 are still acceptable. Anything beyond that, I don't think I would go there unless I didn't have another option. ISO 3200-6400 is the equivalent of +18dB on a traditional video camera. Hideous. Even at low ISO's you can still see where the codec is having trouble resolving very rich and saturated colors like deep red and purple. I must say, I'm pretty impressed with this as compared to the 7D. I didn't have a chance to do this test with it but even at ISO 400 I noticed a lot of noise in dark tones.