A simple but convincing line on this debate is from the Huffington post, via a NY Times blog.
Over the past four years I’ve asked police officers throughout the U.S. (and in Canada) two questions. When’s the last time you had to fight someone under the influence of marijuana? (I’m talking marijuana only, not pot plus a six-pack or a fifth of tequila.) My colleagues pause, they reflect. Their eyes widen as they realize that in their five or fifteen or thirty years on the job they have never had to fight a marijuana user. I then ask: When’s the last time you had to fight a drunk? They look at their watches.