I can't speak to how honorable any of them are but Carshield is ~$100/ month. I say keep up with routine maintenance and invest the $100/ month.
All insurance/warranties companies are out to make a profit. The cost of premiums will cover, the expected cost of the insurance (Work they will do), and the cost of the salesman. Usually the latter is the biggest percentage. I think on average, most insurance will only pay out 15% on average.
So, they are usually a bad deal.