I've been using macs for years... or should I say apples, because I had a couple before the Macintosh line was introduced. I have never bought AppleCare until recently. I bought a dual 2.5 G5. I believe it was 250 bucks for the AppleCare plan... that computer was around 10,000 dollars fully outfited, so the 250 dollars was worth it, in case there was a potential problem, there is a lot of money in that nice aluminum box.
I have not had one problem with the computer, but I still am happy to have purchased it. There is a little bit more then a year left on it. I plan on keeping this computer for at least another 2 years. Hopefully if there are any problems with it, it will be in the next year.
Then about 18 months ago, I bought my first PC, (for work) a Laptop, nothing to great. I have had problems with it randomly shutting off. (Probably overheating although it didn't seem excessively hot) Right after the warranty was up, the monitor started flaking out. Depending on the angle it was opened, sometimes the backlight would turn off. Finally, on Christmas eve, the bottom 1/3 of the monitor died, leaving nothing but a huge blur. Not to mention the second charger stopped working at about the same time. So now I am left with a POS that I cannot even boot up, and even if I was able to, I could only use the top 2/3 of the monitor, missing the task bar.... Now mind you I was very easy on this thing, it never was dropped, never spilt on, nothing...
So I ordered the entry level MacBook, as all I really use it for is browsing the internet while in bed, and traveling, and maybe dumping photos on while on the road until I can load them back on the G5. I decided to purchase the AppleCare for 250 bucks, nearly 25% of the purchase price. But at least this way I am guaranteed a working laptop for the next 3 years, more then I can say for the last one. That alone is worth it in my opinion.
Now I can only think of one time I have ever needed to bring any of my computers in for service in all the time I've been computing, and it was more of a mistake on my part... not a hardware failure.
So for a desktop, I would be more leery about purchasing AppleCare. But when you think about it as less then 5 dollars a month to guarantee it is around for 3 years in good working condition, it may be worth it.
As for credit card extened warranties. Has anyone used them? I have a feeling it would be a mess to handle. At least with AppleCare you call them and send it to them and they worry about the cost and repair. I don't think you'd send your computer to AMEX and they take care of everything. This is one reason I bought AppleCare over the other warranties out there as well. I'd much rather not have to hassle with getting it fixed, and fixed right. Seems much easier to send it to Apple or drop into a Apple Store.