I would disagree with regards to the usability of knobs. They represent the information that would otherwise be presented in a numeric text box or a slider in a much more compact space (since it's angular vs. linear displacement) and the mouse based interaction generally is the same as a slider (grab and then move vertically to change or move vertically with a modifier for fine grained movement).

There are applications out there which use angular drag/mouse information when grabbing knobs, but they are the tiny minority (and as you can guess there's all sorts of issues doing so).