I was at San Jose State back in the 90's (god, that sounds old...), a bunch of us were sitting outside the dorm having a cigarette (glad I gave up that habit) and there was this car that the alarm kept going off on for absolutely no reason. And for like 5 minutes at a time. During one of the times that it was quite, I jokingly got up, went over to the car, said "I'll give you something to alarm about", climbed up on the bumper, and jumped up and down a few times. Damn thing didn't make a sound. We started laughing and I walked back to the steps and sat down. At that moment, a leaf floated down off a nearby tree, landing on the hood of the car. Alarm starts blaring! We were rolling after that.
They were effective for probably all of a month when they first came out. They certainly got people's attention. But then it became apparent that they would often go off for no reason. Repeatedly. Now when they go off, no one ever thinks "oh shit, someone's breaking into/stealing a car". Instead it's "oh shit, some douche's car alarm is going off again, wish he'd shut it up". I honestly don't understand why they even equip cars with alarms anymore. Totally useless.