Passenger vehicles have for third year a 60,000 mile limit and covers most components excluding trim, wear and tear, wiring.
Commercial vehicles have a 100,000 mile power train warranty which covers main engine components and gearbox and driveshafts.
As for experience with extended warranty, I have ten years of experience with fiat warranty, every day of my working week. Normally, like Ian says, the fault for invalidation is with the customer, either by not servicing the vehicle at a vat registered garage, or by getting the service carried out too late. This is not fiats fault, its not the dealers fault, and when a customer realises that their bill is not going to be paid they MAY come on a forum like this, trying to gather evidence to support their case. And they MAY moan about Fiat warranty, in fact they MAY say its worthless.
But for every person who has had a claim turned down there are 20 who have had a claim paid for under warranty.
NumanR