My verdict? Not a strength exercise. It's a conditioning tool. The founder of shovelglove uses a 20lb hammer after years of training, and my bet is that any guy with a 400+ squat (obtainable in less than a year for many young guys) could start off with a 30+ pound hammer without a problem.
Not trying to diss it, I just have to warn skinny guys that swinging a 10lb hammer is not representative of strength.