I bought a G9 to replace my MX518 about a year and a half ago, and I absolutely love the G9, one of the best mice I've ever used. Avid FPS gamer here, and even after 6+ hours of non-stop online gaming (TF2, L4D, Cod5), my mouse hand feels fine. With the MX518 it would start to cramp after 4 hours.
I bought 1, couldn't resist it at this price.
Heads up for those that prefer a more rounded mouse: the G9 is fairly flat, and does not have a round upper body at all. I don't like rounded mice, and love the flatness of the G9, but I have friends that prefer the "rounder" higher MX518 body style, so think about that when ordering.