Since it is likely a lot of us will be buying a new iPhone in the next few weeks and, possibly, a new iPad in the next few months, I wanted to revisit the AppleCare question.
AppleCare Plus on iPad and iPhone - Yes
On mobile devices AppleCare+ makes a lot of sense. It costs $100 and lets you replace your dropped, dunked, or otherwise wrecked iPhone/iPad twice. Granted you will have to pay a fee for each exchange (it used to be $50, now it's going up to $80) you are still bucks ahead on phones and iPads that start at $550 and could go up to nearly $1,000. I don't use a case but think of AppleCare+ as my invisible case. We've already used this on two of the iPhones in my household and the iPad 3 has a nasty scratch on the screen that will result in its replacement as soon as I get time. Unless you've got some other insurance on your iPhone or iPad, I wouldn't think twice about it.
AppleCare on MacBook - Probably
I've always bought AppleCare on my MacBooks as well. About five years ago I had a MacBook Pro that had something wrong inside where it would reject a logic board after a few months. This started about 18 months after I purchased it (6 months out of the standard warranty). After Apple replaced it a few times, and the second replacement failed I received a call that started with, "David, we're just going to give you a new Mac." As a result, I keep buying AppleCare on my laptops even though I've never needed to use it since then. I probably still will continue to buy it with laptops though I can understand why some don't. They are pretty reliable as of late. The best case for not buying it is if you keep your laptop on a desk all the time and treat it more as a desktop machine. Still, there are a lot of densely packed electronics in there and it is so easy for something to go wrong on a machine so complex. Note that AppleCare for Macs does not cover accidental damage.
Apple Care on the iMac - Probably Not
This is the hardest case for AppleCare. iMacs are generally easier to repair and they aren't dragged around with you every day so there is less wear and tear. They generally have better ventilation (and bigger fans) and are much less likely to run into issues. I've got AppleCare on our iMac (that is now approaching three years) but don't think I'd buy it again (assuming we replace that machine).
An insurance guy once told me that lawyers are the best insured people on the planet. We always see things going wrong so we always buy insurance. Maybe I've got a bit of that bias but when I replace my iPhone, I will be paying an extra $100 gladly.
For more on AppleCare Macworld recently did an article summing up all the details.