I would have to say that for the funtionality/money, that the iPower is the best value and holds the longest charge. The new Air Pack is cool in it's design, but it is a little heavier than expected, costs around $80, and it is pretty hard to take off of your iPhone. The iPower doubles your battery life, has an external amplified speaker, costs half the price, and even works as a protective case for the iPhone. I own both and prefer the iPower. For those of you not familiar with it, I bought mine at
http://www.iphonebatterypack.com. If anyone knows of one that's even better, please let me know.