I love power shrugs!
Within 2 weeks of doing them I was doing 315 for sets of 5-8 reps very explosively and MAN was I sore. But just like madcow2 said, I did notice almost instantaneous trap growth! Like a dummy I quit doing them because I got suckered into doing them lighter and sloooow. Talk about garbage! I get a better contraction on them pulling fast and explosvely. Same goes for rows and pulldowns of any kind.