i prefer the Juice Pack Plus from Mophie. For roughly $20 bucks more, you get a case that is pretty much the same size and weight as the Air but it offers actual protection to the phone. Plus covered buttons though that is personal preference.
The big thing is that the case offers a full charge and then some. The Air won't fill it up completely. About 70% is what I averaged with it.
The Air is a good case. You really can't go wrong with either Mophie. I own them both plus a few others (Incipio offGrid, Energizer, and Kensington PowerGuard). The Mophie cases perform the best, not just in terms of power but also phone usability.