In general this is probably more useful for touch devices--the paradigm of a knob is much more realistic when you're using your fingers. Though it would make a nice progress indicator regardless of touch.
The paradigm of a knob isn't that great at all. I think it's only because of the size contraints (you often need many controls) that you see knobs being used over sliders.
A slider instantly tells you the current level (relative to minimum and maximum) and is very easy to adjust.
Not even then. Touch interfaces pay no attention to the rotation of a single point, so this would require two fingers, and if even then, you couldn't rotate it more than 180 degrees (try it without lifting up your hand). Scrollbars seem to win as far as touchscreens are concerned. Also, it's dependent on the visual indicator. For example, to set something at 50%, you need to move the visual indicator to the 50% mark. So, you'd need to first determine whether the indicator is clockwise to the mark, or counterclockwise, and rotate in reverse accordingly. Contrast that with a slider - if you want it at 50%, no problem, just put your finger at the 50% mark and you're done - muscle memory can help with this.
