I use the Mophie Juice Plus 2000 mah because it looks good and does work as advertised. I only use mine though when I know i am going to have to use my phone alot for business that day...emails, SMS or iMessage and so on. Otherwise I use a Seidio Surface case and holster but the abgove case is great with the company having good CS as well.
Just got the Mophie plus about 2 weeks ago - LOVE IT. Have not taken it off since I put it on. (I usually changed cases every couple days) I like knowing that if I'm running low on battery I will have the Mophie to back me up when i first put it on, it seemed heavy and thick. I quickly got use to it, and it feels just right!!
I will be ordering another one in white this week.
I bought the mophie juice pack air for my iPhone 4 about a year ago. It was a life saver. A month ago, I purchased a juice pack plus for my iPhone 4S and it's fantastic. Still saving my life and feels robust in the hand. Last time I charged my phone full from the case it went from 10% to about 95-95%. Highly recommend it.