Knowing some physical techniques increases the range of possible self-defense options, but the decision to choose a physical option must remain with the person in the situation. Choosing a self-defense class is a serious decision and is preferably based on some research. Responsible self-defense training requires control. It is important that each student in a class is able to control her own participation in the class and never feels forces to participate. Any device is useless to you unless you understand how to use it, and you have it in your hand ready to use at the time of the attempted assault.
None are foolproof.
Realize that anything you can use against an attacker can also be taken away and used against you. While some of these devices have sometimes helped women to escape to safety, it is important to be aware of their limitations and liabilities. Paying a lot of money for a course does not mean that you automatically get better instruction. On the other hand, don't assume that all programs are the same and just go for the cheapest. It is always beneficial to be an educated consumer.