A halfway-competent burglar is going to be in and out of your house in less than a minute, which means that he (there are some female burglars, but males outnumber them heavily) can operate knowing that an alarm system isn't likely to get him caught. The most effective part of the alarm system is, therefore, the signs outside, as they show a burglar that breaking into your house will be *slightly* more difficult than breaking into someone else's.

As I have noted elsewhere, though, a dog makes an excellent theft deterrent system. Alarm systems may be of dubious utility, but no burglar wants to break into a house only to be confronted by a snarling ball of nails, teeth, + fur.

I'm sure someone will mention the old 'house protected by Smith + Wesson' saw. It's worth noting that a handgun kept in the house for self-defense is far more likely to be used to injure another resident of the house than it is to defend against an intruder. And, lest anyone accuse me of being a gun hater, I'm an NRA-accredited Sharpshooter, 1st bar, meaning that I could, if I wanted to, give a burglar a shave before he even reached my front door.

-Z