What's your favorite iPhone 4 / 4S battery case?

Since I've recently started using a 4S, I've found that it's all but impossible to get through a day of normal use without a midday recharge, so I've taken to using a Mophie Juice Pack Air (which our own Thomas Ricker reviewed back at Engadget).

Those of you who are using battery cases: what model do you have? What do you like about it? What do you hate?