My 18 month old MacBook air recently started to slow down ( to the point of being unusable). I have AppleCare and took it to the apple store, they ran diagnostics on it and it passed all, however the suspected the hard drive may have an intermittent mechanical fault. They are changing it for a new one (and transferring all the data etc). No additional cost other than the original outlay! So in my case it has pretty much paid for itself in this one incident. Any other problems in the next 18 months and I'm in credit :-)
I must admit it seems an easy to use, no hassle warranty!
In my case it has been worth it!!