My own personal experience with OnStar. Nothing dramatic but it was good.
earlier this year I was in an accident with my 2002 Z06... very close to the house... I took out a light pole... I called the insurance and a tow truck company.. after that I call my wife. She comes to see if I was ok and we decide to call OnStar after a little bit. OnStar sent out a tow truck and it got there WAY before the other one ever showed up.
I don't know how they do it.. but I love OnStar.