Quick review:
I've been using the Air for a few days now and must say that this is really the only Mophie that should exist. The Helium is just so awkward after using Air. The Air's buttons are perfect, and the overall shape is perfect in the hand. The case is a little big, but that is to be expected though the intersting thing is that since the back is curved 3GS style, it still feel compact in the hand. Also because of this the headphone jack doesn't seem to be as deep and my standard headphones are actually able to be plugged in just fine without the included adapter.
Battery life on the phone went from 9% back to 100% and still had a little juice left on in the case (didn't test how much but one light was still lit on the case). Much better than the Helium which when I tried from 6% on the phone it only got me back to 84%.
The covered buttons also make a huge difference. The button cutouts on the Helium are just too deep and overall make the buttons on the iPhone uncomfortable to get at. The buttons on the Air are a little small but work great. The silent switch also works pretty well.
Overall it is a great case. Slimmer and lighter than I expected (seems to be on par weight wise as the Helium which is just odd). The final note is that the case actually seems like it will protect it. Outside of scratches, the Helium didn't seem like it was going to protect the phone much. The Air seems like the Juice Pack Plus from the iPhone 4 in terms of protection which is a good thing.
If you need a battery case my advice is to avoid the Helium and grab the Air instead. The extra $20 is worth it for this case over the Helium. Good protection and awesome function so far.