Hehe! I used to work for the owner of one of their competitors and I can tell you to carefully read the contract and the various degrees of coverage.
Do your homework. Find out maximum service response times and watch the "Exclusions" for pre-existing conditions. It's been a few years, but they will often try to get out of replacing appliances by calling the problem pre-existing, or corroded, etc.
I don't want to scare you off because they're great when your heating or a/c isn't working, but do be aware of the stuff they don't mention. Good luck!
EDIT: Here's a link I found which, although I didn't read, a quick scanning indicates pros and cons...
http://www99.epinions.com/finc-topics-Homewarranties