If you don't know about Onstar, you don't have a GM vehicle. "OnStar is a subscription-based communications, monitoring and tracking service provided by General Motors. It is a standard feature for all new GM vehicles sold in North America as of 2007 (from Wikipedia)".
So what's the problem?
Everyone wants to be a monthly. Once someone takes a cost and adds it to their "Monthly" pile, acceptance and lethargy sets in. For some people, cancelling a monthly feels like as much work as getting the monthly in the first place. The problem is that you may be spending a lot of money that you shouldn't.
It's easy to get nailed by monthlies and people by default forget that they're not fixed. Make sure that you not only evaluate the monthly bills that you have, but be very cautious about adding new ones.
Onstar is a monitoring system that keeps tabs on everything your car is doing. While this is great if your check engine light comes on or if you airbag deploys, what if you have an accident and the records from their "black box" are used against you in court? If you never hit the breaks before the crash, was it that you were asleep, under the influence, medically incapacitated? Even innocent information can bite you when used properly by a clever lawyer.
What about when you're speeding? Every normal person goes over the limit at certain times like when passing really slow drivers on two-lane roads or just keeping up with traffic on the freeway. What's to keep the government from mandating that Onstar and similar must report to authorities when anyone exceeds the posted speed limits?
What if you are parked near a crimescene and therefore become a suspect? How will you explain to your neighbors why the SWAT team attacked your house in the middle of the night and dragged you away in cuffs? The police aren't in the habit of apoligizing for mistaken identities and even if they did, would that weigh as heavily in your neighbors' minds?
Not buying the "accidental suspect" angle? How about this: Onstar is testing an advertising system that will lower your music's volume and play a commericial for a nearby vendor. If you pass by a coffee shop that's an Onstar advertiser, you might be deluged with coupon notices or some plea to stop in for a hot steamy cup. With a minor programming change, it could even address you by name…