Well, not that it's a typically outcome, but I had my Rev. A MBP replaced a year and a half after I bought it due to numerous issues. Without AppleCare, I'd still be struggling with the old one.
On the other hand, AppleCare has done nothing for me on my 12" PB.
It's decent insurance for laptops, esp. ones with major revisions (like Rev A's). It's less useful, on average, for desktops.