It happened in the parking lot of the Santa Clara Swim Center, where I work out a lot. A woman collapsed and bystanders started yelling for help, and I had just parked my Suburban, so I went to help. She had no pulse, so I grabbed my AED and EMT jump bag from my Suburban, and myself and an off-duty ER nurse did CPR and used the AED to shock her back into an irregular sinus rhythm. By the time the paramedics got there, her pulse was back to 64.
The AED I have (Zoll AED Plus) has a small EKG display on the front, so you can get an idea of what is going on. AEDs will only deliver a shock to a patient when they are in certain kinds of cardiac arrest. They analyze the heart rhythm to figure out if a shock is appropriate, and they tell you out loud if a "shock is advised".
http://en.wikipedia.org/wiki/Automated_external_defibrillator