I mentioned this in another post but I'll give it a proper home here.
Now that knob creation is available, I HIGHLY recommend using the freeware program Knobman.
Grab it here: http://www.g200kg.com/en/software/knobman.html
If you can, use the new Java version instead of the regular. It has a much better inferface.
So why Knobman? Well since Xion handles knobs via animations, you need a method to quickly create a ton of frames. Knobman allows you to set however many frames you want between min/max and render them out individually ready to drop into your skin. The real guts of the app though are in it's animation functions. It comes with a bunch of fundemental vector shapes which can be animated via rotation, size, masks, etc. You can in fact design a great looking knob from scratch with the app, but I personally prefer to design in Photoshop then import the graphic and add an animated line or marker over the top. Possibilities are endless though.
So there you go. Highly essential tool for any GUI designer's arsenal.
Peace, Lance