From what I have seen, more of these after market warranties are scams that take your money and deny you claims than pay out when you need them. Check BBB, yelp and other sites before you make a choice and read the fine print. Some will deny your claims if you have ANY mods (including oversized tires) on the car even if the issue is unrelated to the mods (USAA's underwriter Assurant being one of them). Some will only cover specifically listed parts and if you have a failure of a related but unlisted part they wont cover.
Better advice might be to put the money you would spend on an after market warranty into an emergency fund. Check this out about the company carmax uses-Warranty Group, aftermarket reviews on yelp.
http://www.yelp.com/biz/the-warranty-group-chicago