As opposed to other external battery packs that dangle off of the bottom of the iPhone, the Juice Pack Air and its "big brother" wrap around the device. This is a distinct advantage for two reasons; first, the center of gravity of the combined battery pack and iPhone is approximately the same as a regular iPhone, so the phone doesn't feel awkward in your hand. Second, the many of the other external battery packs must be used with the iPhone and battery pack laying flat on a surface. Why? If you are holding your iPhone and the external battery pack is dangling free, the dock connector is stressed and can be damaged.

Not so with the Juice Pack and Juice Pack Air. Since they wrap around and almost become part of your iPhone, there's no stress on the dock connector. My wife's iPhone 3G had been damaged by one of the other external battery packs, and as a result plugging it into a recharging dock or using just about any dock connector peripheral is problematic. I'm glad to say that the Juice Pack Air worked perfectly with her iPhone.

The Juice Pack Air provides 1200 mAh of capacity, which is enough to provide up to 270 additional standby hours for your iPhone 3G, let you talk for an additional 4.5 hours on a 3G network or up to 9 more hours on an EDGE network, give you up to 5.4 hours more data use on a WiFi network, or pack another 20 hours of audio or 6 hours of video playback. A set of four blue LEDs on the back of the unit are activated by pressing a small button. Four lit LEDs indicate that the Air has a full charge.

The capacity of the Air is less than the 1800 mAh provided by the standard Juice Pack, so you'll have a decision to make -- do you want ultra-thin and less power, or slightly thicker and more power?

On the bottom of the device are a micro-USB port and a small standby switch. That switch provides power to the iPhone 3G. Mophie suggests that you wait until you get the standard low battery warnings before switching on the power to recharge the iPhone. Doing this gives you the most additional battery life, and the standby switch is another differentiating feature of the Juice Pack Air.

I found that it took between 2.5 and 3 hours to give the lithium polymer battery in the Juice Pack Air a complete charge. The included USB to micro-USB cable worked well with the standard Apple USB "cube" AC adapter as well as a car adapter plug, so recharging the pack in either a house, office, or car is a piece of cake.

Finally, I love the look of the Juice Pack Air. It is the first battery pack I've used with an iPhone 3G that actually looks like it belongs with the phone. In case you're wondering, the Juice Pack Air will work with the upcoming iPhone 3G S.

Negatives:
I have a couple of gripes about the Mophie Juice Pack Air. First, the material that it is made out of is glossy plastic and very slick. On several occasions, I had the combo slip out of my hand -- fortunately I was above a cushioned surface at the time. I'd prefer to have the case made of some high-friction material so it wouldn't be so slick. Even some molded ridges would help keep the Air from sliding out of your hand. This material also picks up fingerprints quite quickly, although you'll get just as many on your iPhone if you don't keep in some sort of case. They're easy to wipe away with a clean cloth.

The other concern I have is that the battery appears to cause a light to medium degradation in signal quality. My wife and I were at a nearby restaurant enjoying a meal when I noticed that my iPhone was showing no bars or 1 bar (below left). Her phone, which was unembraced by the Juice Pack Air at that point, was showing 3 bars. As an experiment, I pulled my iPhone 3G out of the Juice Pack Air. The signal level immediately jumped up to 3 bars (below right). WiFi was similarly affected.
Mind you, this was based on one observation in one location. That's not a good statistical sample for a study, so please take this observation with a large, rock-sized grain of salt. I will continue to monitor this situation to see if somehow the lithium polymer battery is causing signal degradation with the Air.

I would also like to see a screen protection film included with the Juice Pack Air at no cost to provide total protection for the device; Mophie does sell the iPhone 3G screen shield for US$11.95, but that puts the entire cost of the protection / battery pack system up to about $US92.

Cost:
If you're considering the Juice Pack Air at US$79.95, be sure to think about your battery requirements and look at the original Juice Pack, which is available for $US99.95. It provides another 400 mAh of power, although it is slightly thicker and heavier than the Air. It is also lacking the standby switch of the Air, which I feel is a wonderful addition as it keeps the Juice Pack Air fully charged until you really need the power.

The Bottom Line...and the Giveaway:
If you're looking for an extended battery pack for the iPhone 3G, the Mophie Juice Pack Air is a stylish and very usable solution. While it's a bit more expensive than some of the other battery solutions out there, the attention to detail in the design and the way that it almost enhances the already beautiful contours of the iPhone 3G is well worth the extra cost.

We're giving away a Juice Pack Air to one lucky TUAW reader! If you own an iPhone 3G or you're planning on grabbing an iPhone 3G S in the near future, this will give you the extra power you'll need when you're sending those videos to your friends. To enter the giveaway, leave us a comment with your idea of what you'll use all of that extra power for.

Here are the rules and a link to the obligatory legal statement:

• Open to legal US residents of the 50 United States and the District of Columbia who are 18 and older.
  
• To enter leave a comment telling us how you'll use the extra 1200 mAh of power in the Juice Pack Air
  
• The comment must be left before Wednesday, June 17, 05:00 PM Eastern Daylight Time.
• You may enter only once.
• One winner will be selected in a random drawing.
• Prize: One Mophie Juice Pack Air (Value: $79.99)
  
• Click Here for complete Official Rules.

Good luck! Be sure to check out the gallery below for some unboxing and installation photos of the Juice Pack Air.