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I don't buy extended warranties. Most of my 'problems' have occured within a few days or weeks of getting a new and very expensive lens. Of course, most of these problems were self-induced, and not covered by any warranty - extended or not.
Some add-on warranties just extend the life of the original manufacture's warranty and may not be of much value, especially if the credit card you use does something similar.
But, there are aftermarket warranties which add additional protection from carelessness, gravity, and other tragedies not covered by the manufacturer's warranty at all. For some, these may be of more value.