High ISO noise: The 50D's top generally useful sensitivity--that at which scene content doesn't really matter--seems to be ISO 3200. Above that you can start to see detail degradation.
Noise at ISO 12,800 is pretty bad, and that setting seems to be useful only under a small set of circumstances. There are tons of blank or solid red and blue pixels throughout the images, as well as blurring, which eliminates most small details beyond recognition.
ISO 12,800 noise, the ugly: Shadow areas and large swathes of somewhat uniform color--like walls--display a lot of color noise and significant banding that makes them unattractive, even for relatively small prints.
Noise reduction options: You can see the effect of the 50D's various noise-reduction options on this ISO 12,800 sample. Personally, I think standard (3), the default, sacrifices a bit too much sharpness.
ISO 12,800 compared: They're not in the same price class (we haven't yet completed our testing of the Nikon D700), but a comparison between the ISO 12,800 quality of the Nikon D3 and the 50D shows the difference between usable and not-so-usable.
Picture Styles: Canon's implementation is almost infuriating; I recommend you set sharpness to 3 (or more) with the rest of the Faithful parameters and never use anything else. Why? For one thing, because even though they're described differently in the manual, according to the camera, all of these have the same color settings (0 adjustments), so you've got no baseline to begin your adjustments from in the field. Second, as you can see, the other options really mess up your colors, beyond simply increasing saturation.
Sharpness: On the left is the Standard sharpness setting; on the right, it's bumped up to its maximum of 7. Overall, I find the 50D's photos soft, so I suggest cranking sharpness up a couple notches from the default.
