Some companies make an absolute fortune on extended warranties, which is why they are keen to sell them to you.
It is generally better to spend the money on a better make that will last longer.
So, for example, rather than buy a cheap washing machine for say 250 pounds, and spent 150 pounds on a warranty, buy a good make like Bosch or AEG in the first place for 400 pounds or so.
Most devices that are going to go wrong usually do so in the first year which is covered anyway.
Generaly I would say DO NOT buy an extended warranty.