Certain from Discount are great...except,if they have to replace a tire,and the entire set has 30k miles on them. Then you'll end up with 3 tires with 30k miles on them and one brand new one. You always have to get the cert for the new tire too,so its not entirely a free replacement.

Then...when the 3 original tires are at the end of thier service life,you still have one with life left in it. It actually is better than buying a new tire though. They won't honor the cert if the tires haven't been rotated on a regular basis though.

 

My general rule is to not buy insurance unless you can't afford to replace the item (like a house). It's just extra profit margin for salesman.