I have used it, and it saved my butt after a concert! While we waited for the crowded parking lot to clear out, we listened to the radio in my car (without the engine running, oops!). When it was finally time for our parking row to go, my car wouldn't start.
However, I grabbed this out of my trunk and it started up my car in no time! Thank goodness, too, as I think the line of cars waiting behind me would have rioted.