Here is two shots of Canon 5d at 1600 ISO. There is 100% crop from the center of images. I think 1600 is really useful. There is no so much noise I expect, that wonderful. The shot with girl is with 24-105L - 1/50, f4.0, 73 mm, 1600, the another one is 1/20, f4.0, 99 mm, 1600.
Soon I will upload more photos in different location and conditions.