It depends- for example, my wife bought me a Nook Color a couple years ago from Staples, and bought the protection plan. About 3 months ago, it wouldn't start. I called Staples and within 2 hours my wife had an email from Staples with a electronic gift certificate for the original purchase price. I replaced my Nook Color running CM10 with a Google Nexus. I bought another protection plan for the Nexus based on that experience.
There are other products I don't purchase them for (such as video games or toys for my kids), because the failure rate is very low, and I'm not into throwing away money.