When buying a house I would definitely make the seller pay for a one year warranty (pretty standard around here). After that if you save that $5-600 per year plus the service call fee plus the overage they dont cover plus the fact you get to choose who you want then you probably come out ahead. I've learned to fix enough stuff on my own that when I have to pay for hvac or other repairs I cant do then I'm usually way ahead. I have known a couple of people that have gotten a new compressor or other large expense covered during their first year of warranty but not many.

Good luck to you