I assume you mean do they pay for themselves due to saving the owner money on making repairs and having them covered or having to pay for them out of pocket?
The way I look at it, if something on the car is going to need to be replaced under warranty, most likely it will show itself well before the standard warranty would expire.
OCCASIONALLY you will have the exception though. Extended warranties are a huge profit making deal for any manufacturer because rarely do folks keep the car long enough to make it worth the cost.
I've never bought one and have never regretted it either.