Low ISO noise
Photos display low-to-no color noise throughout most of the 5D Mark II's ISO sensitivity range, and there's no visible loss of sharpness until you hit ISO 1600, where there's a slight, but perceptible, softening.
High ISO noise
You begin to see a little of the color mottling that characterizes high-ISO noise starting at ISO 6400, along with some more softening, but for the most part there's no significant loss of detail until you hit ISO 12800. Even then images can remain usable for some types of content. By ISO 25600 you've got serious noise, but the images will serve in an online emergency.
High ISO Noise: 5D Mark II vs. Sony Alpha DSLR-A900
You can see the difference between the two cameras compared at the Sony's maximum sensitivity of ISO 6400. While Sony tends to have brighter exposures with better auto white balance indoors, its noise suppression algorithms aren't quite as good as Canon's.
For shots such as this, the 5D Mark II's ISO 12800 performance is more than acceptable, and allowed me to shoot without flash in a very dim room. (1/125 sec, f3.2, EF 24-70mm f2.8L lens, spot meter)
Although you wouldn't want to shoot at this high a sensitivity too often, the 5D Mark II does a creditable job preserving detail given how much the sensor gain is increased. (1/125 sec, f5, EF 24-70mm f2.8L lens, spot meter)
As with all of the high-resolution cameras, you need a really sharp lens to take advantage of the increased detail resolution. While this could be a bit sharper, I also tend to shoot with the sharpness parameters pretty low; in this case, 0. (1/100 sec, f5.6, ISO 640, EF 24-70mm f2.8L lens, spot meter)
The 5D Mark II is capable of very good color, and it's outdoor white balance is excellent; the gray and white measure neutral here.
I continue to be a bit frustrated with Canon's indoor auto white balance. I expect some warmth under tungsten lights, but the company's cameras routinely pop out Creamsicle shots. (Left as-shot JPEG; right, corrected Raw)