I called my wife's car once and she nearly ran off the road. We got some free minutes with the service. Since then, we have never used it. I have OnStar in every vehicle and use the navigational/information services all the time. We just don't use the phone part of it. Besides, we usually have our cell phone in the car anyway.
One note- Just like cell service, there are pockets where you won't get any OnStar coverage either.