I use
this at home. It's pretty ghetto but to be honest, it does what it's supposed to.
Basically all the one I bought does is have a few wireless sensors that connect to the base station. Base station is hooked up to a phone line (and has a 9V battery backup) and calls the numbers it's got programmed into it if any of the motion sensors are tripped.
The only problem I've really had with them are that the sensors are pretty budget, so they sometimes get "stuck" on. (i.e. it thinks that there's something there when there isn't). Because of that, I have to go walk by the sensor to reset it. I've had one of them die on me, but the motion sensors are so cheap (<$10 each), I usually just go buy another one.
If you're looking for something top notch and reliable, then these wireless ones may not be for you. If you're just looking for something that'll give you a warning when a motion detector gets tripped, then this will work.
You may want to check into how the sirens work though. The kit that I bought works of the power line. (i.e. it's kinda like the power line networks) Hence that means I can only put the siren inside. While it's painfully loud inside, you can't really hear it outside so your neighbors might not be able to hear it.