you have to do something pretty far out there to have the feds come after you. They aren't really interested in prosecuting every small-time smoker, they just don't have the resources.
Having lived near a city in which minor weed possesion is only an infraction (Ann Arbor), this law really does make a difference. Think parking ticket from and city or state police officer, instead of jail and a trial. That's who patrols the streets, and answers everyday calls, so it's a pretty big deal.