Well, personally, I rank those things up there with the extended warranties you buy with electronics and such, nothing but junk. Id be sure to read the fine print, limitations, exclusions and all that.
I would have normally recommended working out something with the seller to either have the stuff inspected or replaced as a condition of purchase, but it sounds like you already bought the house. I would think it would be better to have a professional come out and check the system out and have a really good inspection done and have them tell you what shape the system is in and whether or not they think it may be on the way out or in good shape.
If they indicate its at the end of its life, it may be worthwhile to get one. But again I would pay particular attention to the fine print, any exceptions and exclusions and what if any deductible there is.
