It can be terrifying. I had moments where I just made him pull the car over and switch with me because of the dumb #### he was doing. Told him that driving was more than likely the only time in his life when he'll have the lives of others in his hands and that he better take it seriously because that's not something he wants to live with if it goes wrong. He's gotten to the point that he was pretty good around town and have no problem with him driving. However, he's 18 now, been licensed for about a year and a haf and still give a mumble 'thanks' to myself each time he gets home safely.
For example, he still isn't seasoned enough to do interstate / tollway driving comfortably and ended up on one the other day because that's how Waze had him get from point A to point B. He did it, but I my stomach was dropping while he was telling me about it.