The Brittish Heart Foundation Says:
A defibrillator is a life-saving machine that gives the heart an electric shock in some cases of cardiac arrest. This is called defibrillation and can save lives.
Cardiac arrest is when the heart stops pumping blood around the body. When someone has a cardiac arrest, defibrillation needs to be prompt.
For every minute that passes without defibrillation chances of survival decrease by 14 percent. Research shows that applying a controlled shock within five minutes of collapse provides the best possible chances of survival.
… It goes without saying that they are indeed life saving but you also need to receive defibrillator training to ensure you are proficient with its use. This will not only further increase the chances of survival (from using the AED effectively) it will also make your staff more comfortable in using it!