I've always kept my OnStar subs on all of my GM vehicles, I just generally push for the lowest-end "Safety" one, whichever that is. I don't need help with navigation stuff, or any of the other silly add-ons. But the safety? Sign me up.
One night, many, many moons ago, I got into an awful car accident in my '07 Corvette. OnStar was connected to me before my car stopped moving after the hit. My bag had deployed (the car was totaled).
On-Star: Are you OK?
Me: Yeah, so far. Let me check the folks in the other car...
I got out and could hear cries for help, and quickly yelled to the OnStar operator, "Get help here, now."
The ambulance and fire crew were there in no time. They *ARE* faster than 911. They bypass all the horseshit that 911 puts you through (and those operators have to go through themselves). They connect immediately to the appropriate dispatches and get the right folks moving immediately.
To me: it's worth it. It's faster than a cell phone. Been there, done that, got the bruised rib to show for it.