progressive, if you have a rough idling cam the tone will fluctuate with the engine. It gets higher pitched with rpm. I like the way they sound, only reason I don't have one is because I figure I'd get tired of it.
Call me a noise geek, but I love the mechanical noises of any performance part! If I had the bucks, I'd put a stock car-style quickchange rear in since they sound so cool! My wife's Corvette has a rock-crusher Muncie trans, and I love it so much, I rarley turn the stereo on when we drive it. I doubt I'll put a CD player in my '68, most likely just spring for a decent cam, gear drive and flowmasters.