While it’s probably true that the 11-inch MacBook Air makes for a better all-around laptop than the iPad, if you want to get things done at least, it’s safe to say that buying a folio and keyboard combination will be a lot cheaper.
The adonit Writer keyboard + folio builds-in a a keyboard, so you can easily type away on your iPad. It’s got a quick iPad release and an auto-sleep function for the keyboard as well. It should be available this Christmas for $120 (USD).
I suppose it’s not a bad idea if you don’t have the budget for a MacBook Air, but if you’ve got the dough, you’re probably better just getting the 11.6-inch MacBook Air.