Car alarms are a rip off. They are a colossal waste of time and money because the damn things do not anything. Well, unless you count annoying your neighbors when your car alarm goes off, then yeah, they do one thing well.
The theory is that if a thief tries to break into, or steal your car, a piercing loud alarm will alert the good citizens in the area of the crime. Thus alerted, these good citizens will take up arms to thwart or apprehend the culprit until the proper authorities can arrive. Yeah and I believe in Santa Claus, the Easter Bunny and the Tooth Fairy. Come on, honestly when was the last time you grabbed the 12 gauge to protect and defend your neighbor's car when his car alarm as gone off? The only time thing I want to use a shotgun on is the idiot whose car alarm is disturbing my beauty sleep.
If a professional thief wants your car, he will take it. An alarm, a steering wheel locking device like The Club or even a pit bull will not stop someone who is determined to steal your car. It only takes a few seconds to lift your car and drag it away with a modern tow truck. A thief with a screwdriver and a vice grips can start your car and drive away in less time.
A friend of mine had his car, a 2000 Acura Integra Type-R, "protected" by Lojak, which is a car-tracking radio device. Hidden deep within the innards of the car, Lojak alerts the authorities automatically if the car is stolen and they advertise this feature heavily. Well apparently the same people who install Lojak on your car are not above stealing Lojak-equipped cars because my friend's Integra was stolen, stripped for parts and never recovered even though he paid a lot of money for Lojak "protection."
Junkies need to eat also. If you leave your CD collection scattered over the seats even a drug addled thief knows that he can get a buck a CD at the local pawnshop, no questions asked. If he heaves a brick through your window, he can scoop up a windfall. And if your car alarm should scare him off without the loot, you still have a smashed window.
There are only a couple of truly effective ways to protect your car and its contents. First, do not park you car in a questionable spot. A dark alley, behind a beer bar is not a good place to park your car. If your neighborhood is questionable, it is time to move to a better neighborhood.
Second, keep your valuables out of sight. This seems logical enough but if you have your CD collection stored in one of those sun visor holders, you are asking for someone to make your collection their collection.
And maybe the best theft deterrent of all, a hidden kill switch. This is a simple on/off switch in line with the fuel pump's power supply that can be hidden so that only you know how to turn on. Without power to your fuel pump, modern cars will not have enough fuel pressure to allow the car to run. Even if the ignition is forced or "hot wired" the engine will not run without adequate fuel pressure.
So do not waste your money on car alarms, save your money to move to a safer neighborhood.