Well, actually they leave black marks everywhere: drum heads, cymbals, even practice pad. Otherwise good sticks, heavy (I like oak) and balanced. Completely unplayable on your own drumkit, if you care about it. However, you can play them on other's person drumkit (lol).
P.S. I actually wonder if it's allowed to try them in a shop ever. They gonna stain any practice pad after couple of strokes.