Wut? A noob fanboy comparison, really.
This Asus is the best netbook on the market. Battery lasts more than 6 hours of constant video playback (tested), not 5 on standby like the Air (which I doubt considering the core2duo architecture and single gpu solution).
There's 1333mhz ddr3 ram in Asus as opposed to slow ddr2 667mhz memories macbooks use, Nvidia Ion (with 512mb of gddr3 memory because it's not "integrated" for Christ's sake) is BETTER for a mobile device than INTEGRATED 320m in Air that also steals your ram to work and slows down the whole system in process.
Also Asus packs a second gpu for energy-efficient usage which makes it last for 7-8 hours.
Also, Asus rocks the fastest Atom dual core to date which in fact is a tad (not 2 or 3 times) slower than a ulv core2duo. We're talking about 20-30% since Atom d525 is almost 3 times faster than first single core atoms.
Not to mention that a physical hard drive being a few times bigger and FASTER and its main downside which is battery usage still makes it last longer. That comparison sucks.
Thanks to Mac's slightly faster processor which is it's only plus except of size (because of smaller screen size in MacBook) these devices at best are comparable and Mac is twice as expensive (or even more than twice if you take 1215n's real price, not the made up from that comparison because by definition it's a sub-500$ netbook which never costed more).
You can also buy a 11,6 asus or even 10 inch asus which will be smaller and lighter.
Oh and if it comes to quality then new Asus laptops are top notch, not plastic piece of crap. I used to dislike Asus but this year they are really good.
To top it off, you can play Crysis on 1215n with satisfying frame rate on medium settings.
edit: And that's comparing a sub-500$ netbook. For 999$ you could get a sub-notebook from Asus that would wipe the floor with MacBook air. For example UL80JT with quad-core Intel I7 (now literally 2-3 times faster than core 2 duo in Macbook) and a 12 hour battery life..