Extended service plans are a gamble. Some are out and out scams (get a bunch of payments and then go bankrupt when the bills start coming in), some are written so it is very difficult to meet the requirements to get payment.
There are 2 flavors, inclusive and exclusive. Inclusive lists what is covered and nothing else is covered. Exclusive lists what is NOT covered, and everything else is covered. Exclusive is much better.
Read the CONTRACT (not a sales brochure) before signing up, and make sure that you understand what is covered, what is not covered, and what you need to do to qualify for coverage.
Based on what I've seen in forums, Good Sam is less obnoxious than many extended warrantys.