Apparently almost no one. An ex-employee of a client got a job at their 'customer retention center' and was surprised how many people didn't realize they had it for x-number of years. It was never used, and the question 'why would I want to pay for something that I have never used' was common. They celebrated people that re-upped their subscription it was so hard to sell.
Sirius/XM is another service that people have a hard time paying for. They usually use it because the dealers usually demonstrate it for customers, but many (most?) don't resubscribe, but I would think they have a slightly higher retention rate than OnStar.