If you are considering an iMac, I would say that it is worth it. There have been a few reports here in Mac Forums of troubles with the screens. I very nearly bought a new iMac (went instead for this Mac Pro), and very definitely planned to get AppleCare for that very reason. I would recommend it.
For a Mini or a Mac Pro, I don't think it is necessary.
For laptops... I didn't get it when I got my MBP, but I can see that it might have been of value. If anything goes wrong with that MBP, I will regret having not purchased it.