Don't have a S&W with the lock but do own a little Taurus 85 snubby that has one. Never any problem with it. I've locked and unlocked the hammer a few times showing friends how it works but never use it otherwise. I do carry the key along with my others on the ring. I guess in a home with small children it might have some merit for a person too lazy to just put the gun in a safe place away from kids. Personally I think some kids, being smarter that many believe, would more than likely figure a way to get the gun unlocked since it's basically a hex key for the Taurus guns.
In my opinion a waste of money by a politically correct innovation brought about by IGNORANT people and politicians.