There was an article on the IEEE (Institute of Electrical and Electronic Engineers) about people stalking other people using those devices.
The concept is great, see where your teenagers are, if your spouse is straying etc, etc. But apparently there are some bad citizens who are harassing others by using them. From what I read they are easy to install, small and undetectable.
If you are looking for an alarm system to recover a car, look into LoJack. It is installed in a car. You get a serial number. If the car is stolen, you report it. The homing beacon is turned on. The police find your car.
They even make LoJack for laptop computers.