Often such things aren't worth the paper on which they're printed or even less than a pat cliche.
However, my mother had one such Best Buy plan on her computer. When the modem failed about eight months later, they replaced it for her. She did have to take the computer tower to the store and wait about two weeks before it was fixed, but it was fixed.
I suppose it depends on the expense of the item and the confidence you have in the brand as to whether or not those things are worthwhile. ...Though I can't imagine having many problems with phones.
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