Best Buy is especially aggressive about selling you a worthless warranty. I bought a $15 phone a few months ago. I was asked at the checkout whether I wanted a warranty for it. I said "No.", then the cashier asked me why, to my surprise.
For 2 items, Consumer Reports recommends buying an extended warranty, one for plasma TV and the other for Apple computers. I'd add that if you buy a laptop, you should get the best warranty you can afford. Repairing laptops is very expensive. Laptops are much more susceptible to damages than people think. If you ever need to make one repair service call during the warranty period, you get your money's worth.