If I needed any evidence for the uselessness of a car alarm, I received it early this morning. At 5am, some neighbor's alarm goes off. The thing doesn't get shut off for 20 minutes, more than enough time to get away.
It was probably some cat or something that jumped on the car, which means the alarm is too sensitive. If you want to secure your car, there are better ways than using an alarm. And less annoying, too.