I personally have never bought it because I tend to buy cheaper, easier to replace machines and have had good luck with them.
But a macbook air is much more premium machine so I think its worth looking into. You have a year from the the date of purchase to take it out and it will extend you upto 3 years of coverage.
I used to work in an apple store, and the amount of times I saw people bring laptops in just outside the one year warranty and hit with expensive bills because they didnt have applecare was always horrible to handle.
You can shop around and get AppleCare cheap from other sources than Apple itself. I think personally that its the only way you can 100% guarantee have a macbook air for 3 years (hardware wise).
What I always said to customers buying expensive machine who couldnt afford applecare at the time was to save for it and buy it before the years warranty is up, gives you quite some time then