If you do, shop ONLY for a GM service contract. Why? Well the problem with 3rd party service contracts is finding a repair center that will honor them. GM dealerships are usually under contract to work only with GM contracts. Ford with Ford, Dodge with Chrysler, etc. I worked at a dealership for a while, in the service department, and please just trust me, this is not one time it pays to shop around!! Every week we turned away good people who purchased these 3rd party service contracts because of the terms the dealership was under with the manufacturer. We could not honor them, and it is entirely up to the customer to try to find someone who will.
There is nothing wrong with a good service contract/extended warranty, and sometimes just in 1 repair they will more than pay for themselves, like if the transmission needs replacing for example. Be sure you include towing, and also a loaner vehicle if you might need one.
Back to the 3rd party contract: the transmission blows, and eventually you find a garage that will take the coverage for you. Its 125 miles away. You have to get your truck there, and no, towing isn't included. They replace the transmission, but because authorized service at a dealership did not do the work, there is a very limited warranty given to you on the replacement transmission. Parts only, 6 months or 6K, 12/12 if you are lucky. They install a rebuilt transmission from a 3rd party supplier rather than a new one, because the fine print says they can. The fine print also says that once they install the transmission, their obligation is fulfilled, regardless of the milage on your vehicle. In other words if transmission problems start 15K later, you're on your own buddy.
This is a pretty typical account of how it goes with those 3rd party contacts.