On some equipment it is totally uselss. PC parts are obsolete so quickly its worthless. Same for most electronics.
Some home appliances it is useful to ahve one but you have to make sure that it covers useful/likely problems.
There are exceptions. Everyone I have talked to at the repair shops (the outside of the store ones, too) has told me that it is an absolute necessity to get one for a big screen TV. As almost any problem gets you a replacement (often better than your original) and they all have a lot of problems compared to the old CRT units.
Cars are ususally useless for the extended warrantees as well.
Anything serious will show up early and as long as you do regular maintenance you won't need anything done until well after any possible warranty has expired.