I saw a mophie helium juice pack at Best Buy today as well. I prefer to carry one of their small external chargers in my purse so my phone isn't always so bulky. This was almost half the price and has nearly double the battery capacity as the mophie juice pack:
It was in my local Apple store in Fort Lauderdale. I wasn't too impressed. The flash is now too recessed so its quality is compromised. The headphone jack now requires an extension which makes it more bulky. I agree with Europa and prefer an external charger. Regrettably I don't carry a purse.
I just returned one yesterday. I really live the concept but it still feels bulky and for some reason it's not comfortable to hold in hand. Especially one hand operation. The surface is too smooth that's feels the phone will slip off my hand in anytime. Just not for me.