I use it for my 13" Pro Retina....and I like it but it is a little loose but fine. My guess is it would be a little loose on a Air...so if you don't mind the extra room it will be fine. If you want a snug fit, this is probably too big.
It is not QUITE as tight as I would like on the 13" but it is very, very close and it works just fine in a backpack, etc. I really don't think a 15" would fit in there without really, really stretching it.