Comparison of ISOs ranked low to high in order of noise (based on the 6D). Because this was ordered the way that the 6D shows noise, the 5D Mark II has some that are out of order. All video files had a +50% midtone exposure increase to help show difference.
Definitely was surprised with some of these results. All of the video coming out of the 6D is significantly cleaner -- especially once you get into high ISOs. The noise has a much finer pattern on the 6D compared to the massive blots and chunks of color you get in older DSLRs. I never went past 1600 or 2500 on the 5D Mark II if I could avoid it, but on the 6D you can approach 12,800 without having to worry too much about image degradation (depending on what you're filming, of course). The expanded ISOs are a bit messy, naturally, and you start to lose a little dynamic range if you push it, but if the option is get the shot or don't, the 6D is a fantastic low-light option, especially paired with a fast lens.
Now we can shoot video indoors with Canon's f/4 zooms without them looking horrible!