I haven't plunked down the $$ for the applecare on my october unibody MBP yet but I plan too. I found that on a laptop it generally makes sense for the extended warranty. Given the type of usage that occurs with laptops. There's a higher probability of failure due to the abuse of moving it around, in bags, planes etc and the heat from such a small enclosure.
plus the cost of a single repair far exceeds the cost of the warranty in of itself.
Also consider resale value, if you typically sell your laptops within the 3 years. People generally pay more money for a laptop that has applecare then those that don't.