the only thing that bothers me about onstar is HOW much it is integrated into the car.. i mean, you can't put an aftermarket radio in without additional cost or your airbags might not go off. They can shut your car off, they can lock you inside the car, and they can change your radio station, etc.
GPS gives an approximate location and thats fine. Onstar can sell information about you like what radio programs you listen to, how fast you drive, what times of day, if you wear your seat belt. How often you change your oil, what routes you drive, ETC.
It seems like a marketing scam.
It also seems like (if you like the big brother thing) that they could call a police officer sitting on the side of the road, and say "_______ is traveling at 9 miles over the legal limit, is not wearing a seat belt, and is listening to death metal, when he passes you we will shut his car off and lock his doors for you."