Nope, never. Which really cheeses off the sales people, because some places make more on the extended warranties than they do selling the products.
Last instance was a washing machine (cost £239). AO wanted £6/month direct debit for their super-duper protection (racket). That's £72/year, and you have to start paying immediately.
So not only do you have to pay for the first two years which have a manufactuers guarantee anyway, within 3.5 years you've paid more than the cost of the machine. If you keep paying that into the 4th, 5th years (or longer), you've been completely ripped off.
So the only way you'd actually benefit, would be if the machine broke down between year 2 and year 3.5. That's a window of 1.5 years where having the cover would save you money in the event of a failure. Even then you only save money if a failure occurs, and that obviously is a gamble.
So in that case (and pretty much every other case I've seen), you're better off saving the money yourself.
e: even if your machine fails just after its 2nd anniversary, you only save £100. You've already spent £140 on the warranty payments. And if your machine fails after its 3rd anniversary, you've paid £210 in insurance payments, and saved only £20.
To my mind it's not only a con but an obvious con.