I don't have any specific knowledge, but here are my thoughts on alarm systems.
An alarm system can only do one thing: notify people of a break-in. Some systems only notify you, others also call the alarm company who may call the police. The top-of-the-line systems include microphones in your house so that the alarm company can more easily make a decision about whether or not they should call the cops, and what they should tell them.
So, how useful is that?
Well, if you are armed and capable of responding to the intrusion, an alarm that notifies you may give you a few seconds of additional response time, in some cases. In the example from your neighborhood, it's unlikely that the alarm system would have provided any useful notification to the occupants that wasn't given by the sound of the door being kicked in. A good dog would probably give you more notification, and might deter or delay the intruders as well.
There's also the consideration that an alarm that sounds to notify you will also be heard by the intruders. With some intruders, that may make them turn around. With others it won't. If it's loud enough that your neighbors might hear, it's a bigger deterrent to sticking around, because the neighbors might call the cops. You can get the same effect by buying a few inexpensive sirens which you can activate yourself.
The alarms that result in a call to the cops are of varying value. Police get lots of false alarms from these things, which often makes them ignore the calls, or at least de-prioritize them. The alarm systems that enable the alarm company to listen in (I guess some may have cameras as well) allow the alarm company to provide the dispatcher with a better idea of the nature of the situation.
In the example in your neighborhood, an alarm system with microphones/cameras probably could have gotten the police there within minutes. Even better, I think they have speakers which would allow the alarm company to tell the bad guys that the police are en route, which might get them to leave more quickly -- which is a good thing.
An alarm notification without active monitoring, though, might not get a high priority, so the police may have arrived quickly or they may have gotten there an hour later. If the system played a recording that told the BGs the police had been called, or if the occupants told the BGs, then it might encourage them to leave. Of course, you could tell the BGs the police have been called, and if you have already triggered a siren they'll probably believe you.
Unless the BGs manage to get inside and overpower you before you're even aware they're there, you can call 911 and get the police en route yourself. Even if you don't have time to talk, a dispatcher who hears screaming will get the police there ASAP. If you use a cellphone, you can defeat attackers who are smart enough to cut your phone lines before entering. It's worth pointing out that an attacker who cut the phone lines would stop your alarm system from calling -- and you might not even realize that the call didn't go out so the alarm system might actually work against you in that case, making you think you don't need to call 911 yourself.
Bottom line: In the event of a fast, violent entry an alarm system isn't going to give you much notice. A dog will probably give you a little more warning. The alarm system might call the cops, or alert your neighbors to call the cops but unless the intruders are on you really fast, you can call the cops, too.
In the case of a slow, cautious intrusion, sneaking in through an unlocked window or such, the alarm system will be useful, but a dog would do a good job as well. If it's the right kind of dog, it can be an active defense in addition to a warning system.
IMO, unless you have money to burn: get a good dog, heavy interior doors in good frames which you lock at night, have a phone in pretty much every room and/or keep a cellphone on you, deploy plenty of motion-activated lights all around your property -- I think you can get some that also have a remote control so you can switch them all on -- and maybe think about putting a loud siren in each bedroom. Also, keep your windows and doors locked pretty much at all times.
If you do have money to burn, do all of the above and then get an alarm system. Whether or not it has active monitoring depends on how much it bothers you that the alarm company employees can listen in on you (Hmmm... bribing an active-monitoring company employee to tell you when a rich house is empty might be very useful if you're a burglar) and on how much money you want to spend. Active monitoring has a monthly fee, plus a greater up-front cost.
Oh, and obviously be prepared to defend yourself, because even if you or your alarm system call the cops, they'll still be minutes away.
Just my two cents.
