My insurance recommended I get a Tag stolen vehicle tracker and my 1st year discounts paid for it. I did not need that service from OnStar anymore and cancelled.
That said, it depends on your needs. They have many packages and options. If you're on the fence, when you call to cancel, they'll probably offer a limited time discount. It will give you a few months more to see if you use any of it. Just remember to put a notification in your calendar before they bump you back to full price.