Getting into garages is quite easy and takes 6 seconds. The bad guy simply takes a wire coat hanger, maneuvers it between the door and the flange trim, hooks it on the emergency release and pulls.
Put a deadbolt on the man door going from the house to the garage. Also - NEVER leave your garage door opener inside your car. A friend of mine had her car broken into - the thief stole both her opener and her registration (which had her address) and burgled her house while she was at work.