Aaron Adams in The value of AppleCare gives a great example on why Applecare pays for itself.
I agree, AppleCare is invaluable if you have a portable. I’m not as sold on AppleCare for desktop machines though. Desktops aren’t subjected to the same torture of being lugged around in bags, being dropped, being carried by one hand, etc… My office now has AppleCare on every laptop we buy, and I think well over half of our laptops have had to go back to Apple at least once in their three year coverage period. The iMacs and Emacs are running along fine, no AppleCare on them.