A few weeks ago I purchased the Mophie Juicepack Air in preperation for my trip to Chicago for my class reunion. When I announced on Twitter I was picking one up I had a few requests for my thoughts on it after some use. Admittedly, the new iPhone 3GS has substantially better battery live than the 3G. Very noticeable. However, there are times and situations where that's still not good enough. I thought forward about what my situation would probably be. I'd be flying on a plane listening to music/watching videos (no way to charge), then getting off the plane and going downtown (no way to charge), then hanging out at the CenterPost office (not charging), then heading out to wherever I was going to end up for the remainder of the day (maybe not charging). And not knowing where I'd be staying one night to the next I knew when I had an opportunity to charge I had to make it worth my while.
So the Mophie Juice Pack allows you to double your battery life by building a battery into a hard case for your iPhone. You can turn it on and off at will, and I think that's really key. When you turn it on it starts to charge your internal iPhone battery.
Knowing gadgets and charging like I do there are times and places to be charging a device. Inside your warm, confined pocket is not one of those times. Not only does the device head up when charging (like almost all electronics do), but it can't be good for that to happen while against your body with no way for it to exchange heat to the outside world. So I made sure only to turn on the Juicepack when it was on a table, or sitting next to me, or I was watching a video or something. I never just turned it on and then shoved it in my pocket. I don't recommend you do either.
The only drawback I see is the cable that's used to charge the Juicepack (and therefore your iPhone as well). It allows you to still sync your iPhone to your computer while the Juicepack is on, so that's cool. It charges both the external and internal battery at the same time. That's cool. But it's some lame proprietary cable. That's not cool. It looks exactly like a micro usb, but it's just a little different. So if you lose it you have to go buy a new one from mophie for $10.
Was it worth the $80 or whatever? Yeah, it really was. Now I keep it with me in my bag, fully charged and I can take it out any time an emergency arises. I haven't had to, but you never know. It'll happen.
It's a great device, and it's built well. Adds very little extra size to your iPhone. So if you're willing to spend the money on it, and you know you could use it, I wouldn't think twice about recommending it to you.