I don't buy these third party warranties, which are actually not warranties but are service contracts. Remember - these companies make money so they tend to pay out less (much less) than they bring in. Its the nature of the beast and, like insurance, you have to decide how comfortable you are self-insuring. Also, remember your dealer has an incentive to sell this plan - they will get reportedly up to half the premium.
As mentioned above, double-check the coverage and also who can do the work especially if you are traveling. If you like the coverage and feel that Cornerstone is a good company and the service contract is a good price - then buy it.
But if you are willing to look around, I hear decent things about the Good Sam Plan and their roadside policy.
DW, 3 Kids and our Goldens
2012 Shamrock 233S
2008 Toyota Sequoia 5.7L 4WD