A friend of mine used it to track his teenagers kids. At any given moment, he knows where the car is.

The device itself is not expensive but he pays about ? $25/month for monitoring fees.

I personally don't like to do this, in a way, it is invasion of the kids' privacy, but OTOH I understand why some parents do it. I am OK either way.
Another $300/year out of the window if you go this way.

BTW, this is a useful tool if the car is stolen, however, most thieves these days are smart, once they steal the car, they make sure they remove the "Lo Jack" device as some of them are mounted in the engine compartment or even underneath the car.

__________________
1998 E39 528i 5sp MT
2006 E53 X5 3.0 6sp MT