UPDATE (11/23/10): Unfortunately it looks like Monoprice no longer offers the super cheap and super awesome chargers. My apologies to those of you coming here for a good recommendation. If I come across an option that is just as cheap and awesome, I'll be sure to update again. Sorry!
When I bought the iPhone, I really didn't care too much about not having a replaceable battery. "When would I ever need to replace it?" I thought. Little did I know I'd have to fly to CA a few times traveling for as much as 12 hours in a single day.
Honestly I still don't see it as too big of a deal, it would still be a PITA to have to stop what I'm doing and swap out a battery. And since I've been playing games and watching movies on these flights, I'd rather avoid that if I can.
I've already tried a couple of these things... specifically the Mophie Juice Pack Air... which is garbage. Don't waste your money on that.
Just this past weekend I picked up a Griffin PowerDuo Reserve while in NYC (I needed something quick to try to get a little more juice for the rest of the day and it was all that the AT&T store had). I was OK with picking that up without reading any reviews because I basically wanted something right then and there. How bad could it be?
Pretty bad actually. First problem is that it is only 500 mAh. Even the Mophie was 1200 mAh. What does your 500 mAh get you? At most... 25% battery from a full charge. Fuck that.
I'll admit though, that the Griffin I had purchased did have a slight edge in convenience. That is, that particular package came with a regular wall charger for the battery pack which also had a USB port on it for USB charging, but it also had a car charger (also with USB port). So that's nice and all... but I didn't buy the thing to always have to be charging something.
As an emergency boost, I think the Griffin would be fine. Otherwise it is worthless - especially for the money.
The winner and so far best overall iPhone battery pack that I've used? A $10 charger from Monoprice. They actually offer a few of them, but the 1900 mAh version (available in 3 colors) is under $10 and about $1.75 to ship. Lots of win.
Lots of win because you don't need any special cables or adapters. It charges with your existing iPhone/iPod cable and even allows you to also connect the iPhone while it is charging and charge both at the same time. I'm still testing the expected life of it... but here's where I'm at so far: After 45 minutes of charging, I've gone up 30% and the battery pack still reports 1/2 capacity remaining.
And if that isn't enough for you, they also offer a 2200 mAh version (even that is under $15).
For that kind of money, you can't go wrong. Hell, even if it were a disposable battery pack that you couldn't recharge, $10 is a steal if you really need to have an extra battery on hand. Well done Monoprice, well done.