The sobering came from research by the University of Maryland School of Medicine. The school says that many people have an unrealistic idea of how effective CPR is because of the way it is portrayed in television.
When someone has sudden cardiac arrest, the heart goes to an irregular rhythm called ventricular fibrillation, which is fatal unless an electric shock is administered. CPR doesn't stop this. In reality CPR is a stopgap method. It allows the patient to buy some time while proper medical care and machinery arrive and are administered.
This doesn't mean that you shouldn't learn and administer CPR if you find yourself in a situation like this though! Doing something is always better than nothing and you never know how close help is. Knowing this, however, should make you aware of how important it is to call 911 or some other form of care as quickly as possible.