Our only exposure to On-Star was when we purchased our new 2012 GMC Canyon in November 2012. As a courtesy (and to get the customer started), it was activated at the dealer. To introduce us to one of the many services, they had it give us turn by turn directions to our home about 5 miles away. We had other places to go other than home, so I tried to get it to SHUT UP by issuing "Cancel Directions" several times. It never would stop telling where to go... so when we eventually got home, it was satisfied. Never again would I ask it for directions.
We were also asked for a credit card number during the setup so they could do auto-renew at the end of the "free" 6 month period. I kindly declined, but got a bit of guilt trip from the operator on it. I understand that it is their business to make money on these services, but I respond to guilt trips by keeping my money in my pocket.
However, even after the service expires, one can still use the Bluetooth feature to pair a cell phone to the car for hands-free calling. That's a nice feature, but as we use the pickup as our recreational and occasional hauling rig, we don't tend to spend much time in it. Worse yet, I've talked on the cell while heading down the road all of 2-4 times over the last decade.
With today's laws regarding cell use while driving... the handsfree is useful for those that do tend to chat a lot.
For some, On-Star is something they cannot live without, for other like us, it's a waste of time and money, a required additional cost (for the builtin electronics & hardware) on an already way-overpriced product.