Just an fyi to anyone making or thinking of making one of these: Turning a knob with a mouse is the worst interface I can think of. I don't know why audio apps/DAWs fall so hard on skeuomorphism here when the interface just doesn't make sense in the context.
It works great though, what's the alternative? It's visually small, so you can fit a lot of controls in a small space. You can glance at it and know the current setting and where it falls within the range of possible values. By making the mouse control modal when you click on a knob (so you start dragging and can drag over a much larger area than you could for say a slider, which isn't modal) you have immensely precise control over the value in realtime, while still being able to quickly make big changes. This is essential for performance. Combining this with some gentle mouse acceleration for the rate of change of the control when dragging gives you even more precise control. This isn't possible with a slider either.
I would say the opposite, it's basically the perfect interface for a very specific scenario with requirements that don't really occur in much other computer software.