Extended warranties from dealerships are usually a waste of money, but at least they are legit and offer peace of mind. They generally pay out, but even then they can be awfully picky when it comes time to write a check. The 3rd party warranties are usually a waste of money for the same reasons (the amount you spend on them, for MOST people, does not warrant the potential savings from having it), but also they are hugely notorious for being scams, rip-offs and not paying for jack or shit. Also, you can't take out insurance to pay for something that is already broken. They wont insure you and if they dont do an inspection it's probably a scam, or they'll decline you so fast your head will spin. If you conceal it and they find out that is insurance fraud. Also, if you have one of these, and paid in a few grand to get it over time, and get 2 major repairs covered, and then they drop you for overuse, you still paid in more than you got out. I got one once as a throw in from the dealership where they paid for it. Had I not I guess there would've been some merit to having it for the relatively low cost and it was to cover major power-train (expensive) damage for longer than the original manufacturer warranty. That's not a bad idea, but usually not used. If you get the lemon, though, you'll be happy you have it.
In short, if it's from the dealership/manufacturer you should probably walk away or at least comb through every word of the policy and do your math, and if it's from a 3rd party policy (not from the manufacturer/dealer) run as fast as you can. Research any and all companies you do high dollar business with via the BBB and consumer reviews/ratings/insurance ratings/etc.