I prepaid for 4 years of service. I thought it was a pretty reasonable cost and I'm not interested in having to worry about whether something is covered or not covered. I've never had anything but great service with the Chicago service center, and the only inconvenience of having to do this is the driving part. I've had the car serviced now 4 times. Once I got a rental. Once I got a limo. Once I got a Roadster. Most recently (today) I got a P85+. All issues have been resolved to date, and when new ones come up (minor), they correct them.
Can't ask for anything better than that.