I used the pulley for about 10k miles with no slippage or noise issues at all. It is significately lighter than the steel pulley i replaced. I beleive it is made of 6061 aluminum. I did ask the machinist when he made the pulleys about using steel... He has built many, many, many race motors, making many custom one off parts, and recommended using high grade aluminum over the steel due to weight. FWIW.
BTW i actually have two of these pulleys, as I had him make me two at the time. I eventually went with a whipple so don't need either one. Team Jeff installed my pulleys originally.
Oh btw, the surface of the pulley that interfaces with the belt was bead blasted ( or something... i don't recall the exact term) to eliminate slippage of the belt.
as an aside... the billet for gt guys who make pulleys also use 6061 aluminum. Haven't heard a complaint yet about their pulleys (have one myself.. works great)