On average you'll spend more on extended warranties than you'll get from them. The warranties are priced using actuarial data so the warranty company knows how much they will be paying out in claims. They then add profit for themselves and for the salesman. If you self-insure by setting money aside you can avoid paying that profit. Some extended warranty companies will use the fine print to weasel out of paying claims.
I never buy extended warranties. I've made warranty claims on only a couple of the various new vehicles I've bought over the years, and they were small.