If you're an iPhone owner then you've certainly heard of the Mophie Juice Pack. It's emerged as the external battery pack of choice for any iPhone owner needing more untethered power than Apple can provide from its non-removable batteries. But what about all those feisty upstarts? Can they compete in terms of design, functionality, and price? Let's find out. We put two iPhone 4 external battery pack cases -- the Mophie Juice Pack Air and Mili Po
