I bought Apple Care the first time, when my iPod broke beeing about 11 Months old. I did this in the first place, because it was $99 and the replacement of the old one, even though it was under warrenty, would have cost me $50 without having Apple Care. So I thought for $50 I get 1 year of warrenty on a new product of the same type of a product that broke after less than a year. So I thought that was a good idea.
For my first Mac (the first Powerbook G4) I didn't buy it and when it was close to three years old, the Firewire port broke. A repair shop even discouraged me from getting it fixed, because the necessary logic board exchange would have been several hundred dollars. So risking to break it myself I fixed it for a couple of dollars. If I had had it, I would probably have used it, but am not sure, whether exchanging the hard disk, which I did twice, would have voided the Apple Care warrenty anyhow.
Now I have a MacBook Pro with Apple Care, but did not have to use it yet.