It depends on both the Dealer and the Warranty Company..
The dealers warrenty is basically a mechanical failure insurace policy provided by someone like Warranty Holdings. The dealer pays for this on your behalf when they sell you the car and that cost is taken into account in the dealer price.
No as with any insurance policy the insurer will as you say have small print. The classic is this "does not cover ware and tear or faults present at time of manufacturer", well what then does it cover.
If you have a problem and take the car back to the dealer he will assess it then claim the warranty, the warrenty co may say No (if they are shirkers) and you are now dependent on the deaer to make a stand on your behlaf e.g. threaten to use someone else, make a fulls about it being not ware and tare etc.
Both me and some friends haev had good experiances with expensive warrenty repairs, but both instances were from dealers with exceptionally good reputations.
I think the AA sell a "private" warrenty on a similar basis that is allegedly very good.