In the application I'm developing I wanted to use this to control the angle of the brush image used for drawing. I began by setting the range of the control from 100 and using the value as a percentage (a pretty common approach for sliders), converting to the angle (which I'm using in radians in my drawing code)
Functionally it was okay, but there were some user-feedback issues. For one, the circular slider by default increases in the clockwise direction, but in my mathematics-centered application I felt it made much more sense for angle to be treated as it is on the unit circle, increasing in the counter-clockwise direction. (Since my drawing code was written this way, it also meant that the position of the knob did not visually follow the rotation of the brush image). A related issue: the unit circle typically puts the angle zero at "3 o'clock", whereas the NSSlider defaults to position 0 at noon.
One other usability issue involved precise setting, which is somewhat a limitation of the size of the control. A text field displaying the control setting could help the user a bit; however displaying values in radians would require a lot of decimal spaces and perhaps not be as intuitive, so I decided at this point to convert the control to work with degrees, and display this value in a text field.
To sum up, the needs are: 1) make the control increase counter-clockwise instead of clockwise, 2) make the control set 0 to the 3 o'clock knob position instead of noon; 3) map the range from 0 to 360 degrees and display that value in a text field.
