I've been using a Mophie Juice Pack Air on my iPhone for a while now. It performs very well in most ways - with it, the iPhone's internal battery stays at 100% most of the time. This is good for the longevity of the battery, which is good because replacing it is a gigantic pain. With the battery in place, however, I've noticed that the phone has much worse over-the-air performance. Places that have 5 bars with the naked phone have no coverage with the battery in place. This is particularly the case at home, where the cell repeater gives 5 bars in the living room without the Mophie, but only 0 or 1 with it. The same happens on CalTrain just north of San Antonio. And at my office desk.
There are, however, lots of places where I get 5 bars even with the Mophie. So perhaps it can still serve a purpose. But the worry is that if the battery makes reception worse, then it makes the transmitter work that much harder, negating the battery life gains.
So despite this post's title, I guess I can't entirely write off the Juice Pack. But if you are considering it, be aware that you're wrapping the antenna with a layer of battery.