This is straight from the Oklahoma City Police web site.
Alarm systems definitely have a place in a home security plan and are effective, if used properly. The reason why alarms systems deter burglaries is because they increase the potential and fear of being caught and arrested by the police. The deterrent value comes from the alarm company lawn sign and from the alarm decals on the windows. Home and apartment burglars will usually bypass a property with visible alarm signs and will go to another property without such a sign. Some people, with alarm systems, feel that these signs and decals are unsightly and will not display them. The risk here is that an uninformed burglar might break a window or door and grab a few quick items before the police can respond. Also, don't write your alarm passcode on or near the alarm keypad.
Alarm systems need to be properly installed and maintained. Alarms systems can monitor for fire as well as burglary for the same price. All systems should have an audible horn or bell to be effective in case someone does break in. However, these audible alarms should be programmed to reset automatically after one or two minutes. The criminal got the message and will be long gone but your neighbors will have to listen to the alarm bell, sometimes for hours, until it is shut off. If you use a central station to monitor your alarm, make sure your response call list is up to date. Home alarms, like car alarms, are generally ignored except for a brief glance. However, if you have established and nurtured your neighborhood watch buddy system, you will experience a genuine concern by your neighbor. It is not unusual to have a neighbor wait for the police, allow them inside for an inspection, and secure the residence. A good neighbor can also call the glass company or locksmith to repair any damage, if pre-authorized by you.
The biggest difficulty getting to this level of concern is taking the first step. You can take it by calling your local crime prevention unit at the police department. Most police departments in large cities have neighborhood watch coordinators to help you set this up. You should invite your adjacent neighbors over to your home for coffee and begin the information exchange. You'll be amazed how the process runs on automatic from there.
Alarm systems are effective deterrents with visible signage
Alarm systems to be properly installed, programmed and maintained
Alarm systems need to have an audible horn or bell to be effective
Make sure your alarm response call list is up to date
Instruct your neighbor on how to respond to an alarm bell