When it comes to security alarms, it really depends on what you are wanting it for. Are you wanting it to protect your house while you aren't home or is it for "peace of mind" while you are sleeping. I'm a criminal justice major at Washburn University, and in the past I took a class called Crime Prevention. According to my teacher, who specializes in Crime Prevention, and works for the Overland Park Police Deptarment, he thinks alarms are a waste of money, and will actually end up costing you a lot more with false alarms, since most cities will charge you after a certain number of false alarms...and those charges can be up to $100 a pop.
Basically, what he said was this...if you travel a lot and want something to be looking over your house while you are gone, alarms are alright, but you must keep in mind that there is usually a 10 to 15 minute response time delay before an officer actually shows up. That is a lot more time than your average home burglar would take to grab whatever he's wanting, especially if your alarm isn't silent and tips the burglar off that he's set it off. Remember, most burglars "stake" out their target before they go in, so they will know if someone is home or not. If your alarm is on, but they know you aren't home, they know they have a good 5 mins to rummage through your house and still have enough time to run away before any sign of a cop.
Now, if you don't travel, and you simply want it for "peace of mind" while you're sleeping, you're basically just wasting $100's of dollars on the install and monthly fee that you could get what your really want by getting battery alarms like this
http://www.firstalertalarm.com/
My teacher said that if you go through an alarm company, when their alarm is activated, they give you a 1 minute delay before they call you (1 minute to deactivate your alarm incase it was an accident) That's one minute you would just sit there in your house scared half to death because you think someones broken in to your house. You can't call 911 yourself, because security systems are designed to freeze your phone line so that they can call you after that minute is up. Then they have to confirm with the resident that the alarm is not false, and then they alert the police department. So, basically, after the alarm has gone off, there is about a 2-3 minute delay before the police are actually notified, thus making it 12 to 18 minutes before a police officer actually arrives.
Basically, he said you are much better off using something like those first alert alarm things (which is what I use on my apartment door and windows), or a security system that doesn't freeze your phone line, but simply just alerts you of an intruder, if you are using it for a "peace of mind" application. That way the minute the alarm is activated, you can call 911 yourself and save yourself about 2-3 minutes from that delay.
Basically, if you talked to my former teacher about security systems, he'd tell you to spend your money elsewhere (i.e better dead bolts on your front door, break-proof windows on the lower floor, etc.), because you'd just be wasting your money with the security system. Hope that helps. If you have any other questions, just ask.