The way people get their money back on them is when a motor or transmission goes south.
Engine.. More than $ you want to know. Transmission... I priced a 'replacement' tranny when I thought about an extended service plan.. About the same price as the contract. Differentials.. I can rebuild them myself.
Computer... Now there's something to think about.... Just the diag is a spendy option. Wiring and electrics.. you're gambling on that one.
Anything else is handled by me. The rest by insurance, because if she's goin south in a hurry, I'm gonna start leaving it over night at the boat launch! 
-Steve
Just did a replacement reman trans in a 2003 Grand Cherokee... $3800+tax.
Diag for a Check Engine light in the high rent district of Bellevue is $125.00. A new PCM in a Grand Cherokee will cost about $900 or so but it's covered under the Federl Emissions Standards warranty for 8 years or 80k miles by every manufacturer. It's a federal must. You can't charge anyone to replace it, reprogram it, etc. if it's under 8/80.
Most water pumps now adays will cost with labor close to $500.00.
It doesn't take long to recoop the $2k you spend on a service contract, especially if you're driving a full size domestic truck. I don't care what brand it is.
Jackelope has alot of experience with these companies because the cars they sell always break down...
**Just kidding ** kinda.
You don't have to be kidding.
I don't know why a service advisor who is paid full commission pay would ever want to work at a Toyota dealer.

Sounds like Jack has some good advice though.
I'm a lot better with this kind of stuff than I am with the mechanics.
Fortunately a lot of guys on this kind of site tend to be mechanically inclined and some even enjoy working on their cars and trucks. Thats great. We like to sell parts too. For those who don't work on cars or don't have the time or the space or whatever...service contracts can be good.