Any, although Avids have the handy Speed Dial leverage adjuster so you can tune the feel - with most levers you'll just get what you get, which is usually OK.
Yes - any lever. I prefered it with the Avid lever as you can adjust the way the brake works better, but I've used mine on Avid, Scott and Tektro levers.
Does anyone know if the dial inside an XTR lever works in the same way as a speed dial on an Avid lever. If I go for Avid mech disc brakes, I'd like to keep my XTR levers...