I would consider a personal alarm as “another tool in the toolbox”, but would not rely solely upon it to thwart a personal attack. Too often alarms (car, home, personal) lead one to a false sense of security. Most of the car alarms I have heard go off are totally ignored, as they have become too commonplace. An alarm of any type may deter the majority of thieves/attackers, but if they are determined, you must have an alternative form of deterrence/protection.

Pete