On the back, the Juice Pack Air has four blue LEDs that indicate the level of power left in the battery. In our experience, these lights weren't accurate; we were able to charge the phone even when the lights indicated it didn't need any more juice. On its bottom are a USB port and an on/off switch, which gives you the option to connect or disconnect the phone from the extended battery's power. The iPhone 3G charges its internal battery using power from the Juice Pack Air. When plugged into a computer, the phone is charged regardless of whether the Juice Pack Air is on.
Using the included cable, you can connect the Juice Pack Air to a computer through its USB port, letting you charge the case and sync the iPhone simultaneously--negating the necessity of the iPhone's USB cable. In our tests, it took about 2.5 hours to charge both the pack and the iPhone when starting with both completely dead.
Unfortunately, the case comes with only one Micro-USB cable, and the Juice Pack air is not compatible with a Mini-USB port. If you're planning to charge the case at two locations, such as at home and work, you'll have to bring the cable with you, unless you've purchased an extra one. It would be more convenient if the case used the Mini-USB port that's used in most handheld devices, including the original Juice Pack.
Just as Mophie claimed, using the Juice Pack Air, we got about twice the use time on our iPhone as we normally get; but, that's about half what the original Juice Pack offers. Considering that the Air is only slightly thinner than the original, we were surprised their battery lives were not more even.
Still, running Wi-Fi, GPS, and 3G on the iPhone 3G with the Juice Pack Air attached will yield about two full days of power with heavy usage.