I do recommend getting AppleCare on iMacs. For the record, I'm not generally big on extended warranties. I never buy them on my cars, appliances, etc. I also do not purchase AppleCare on my iPad and iPod iOS devices. Or on an iPhone if I had one.
There are a couple reasons for this seemingly inconsistent behavior on my part. First, look at the ratio of insured value vs. the "premium" cost. Any iMac I would buy is around or over $3000, and the AppleCare cost is about $150 if memory serves. My iOS devices top out in cost at less than $1000, yet Apple charges even more for insuring them than they do for iMacs. Probably because people damage them. I take good care of my iOS devices and, in the event that I do really screw up, feel fine with self-insuring for a loss of a few hundred $$.
Another consideration is that the iMac is pretty much a non-repairable device for the average customer. It is highly integrated - you can't even get inside the dang thing without messing around with special adhesives, etc. So pretty much any repair, even for an inexpensive component, tends to be an expensive proposition. And, of course, a failure of the logic board, screen or GPU would cost more than we want to think about. It isn't like an "old-fashioned" computer where I can swap out drives, GPU, power supply, monitor, etc. for just the cost of the new component. Don't read this as a slam on Apple's reliability. I think it is tops for consumer-level computers. But there will eventually be failures for some people.
That is why I choose not to self-insure on an iMac. I don't have a need or a desire to own a MacBook, but would probably also purchase AppleCare on that since is has comparable levels of cost and integration.
If you are on the fence, this may convince you: With AppleCare you get free phone support for 3 years. I have not yet needed an hardware replacement but have used the phone support several times to resolve system and software issues. It was fast, efficient, and very helpful.
Also, with AppleCare on an iMac you get the same warranty extended to Apple peripheral devices purchased during a very generous time window. Think items such as AirPort routers, DVD drives, Keyboards, etc.
So, all-in-all, I think it is a pretty good deal.