I have the Basic Free Service on my 2012 Sonic: I also have the (included) 5 year plan on an Equinox and a Malibu. I like the ability to remote start the automatic trans cars, and the lock unlock and feature with the My Chevrolet smartphone app for OnStar. You may also opt for monthly diagnostics sent to your email. There are individuals that are paranoid about having any of their actions monitored, but personally, I figure that I might as well enjoy using the built in features in our vehicles that we already paid for.
What I've done in the past is called up their Retention Department and tell them I'm canceling. They usually offer you plans substantially discounted to try to keep you. You can Google "On Star Retention" and you'll see threads on Fat Wallet and the like discussing it. I also did it with XM Radio as well.
I think if it's free service it's good to have, the only thing I found useful is you can unlock the door through the mobile app which is good while you left the key inside the locked car, other than that nothing is really necessary plus the monthly fee is kinda high to me since most of the feature you will never use.