I never purchased them in the past. However in the last five years I bought two that have been worth their weight in gold. The first was a high end refrigerator that I got from Lowes. The extended warranty was less than 4% of the total cost. One month out of the factory 1 yr warranty the refrigerator stopped working. Service came out on a holiday weekend and swapped out all the electronics. It ended up not fixing the problem and the following week I got a brand new replacement. Even better was they reset the warranty to start over again. My Dad has a little lower end refrigerator like mine and had a simple repair that ran him $800. Definitely money well spent.
The other was I purchased a car that was just coming off lease. For 2k I was able to extend the bumper to bumper warranty for 7 more years. About two years later, which was just over the 5 year factory warranty, I had a major transmission problem. Not a penny out of pocket for the tow or repairs. No doubt the total repair cost was more than I paid for the warranty. If anything else goes wrong it's just more money in my pocket.
I think on low end electronics or where the warranty is in excess of 5% of the original purchase price you are better off passing.