I bought the Mophie Juice Pack Boost in an Apple Store in Indianapolis. I just bought an iPhone 4S and have found that I go through about two full battery charges a day (so far). My general impression is that it was a good buy because it does charge my iPhone quickly (appx. 1.3% per minute) but it is awkward to keep connected while on the go. because of the rigid structure of the 30 pin connector you can't just connect it and throw it in your pocket or bag, it really does need to be in a stable location or at least flat in a pocket that won't allow the devices to separate. I think if the device had a 30 pin cord instead of a pivoting rigid connector that the device would be much more functional.
Pros:
will add an almost complete charge from 25%
charges faster than other portable charging solutions i have tried
nice design that matches the iPhone 4(s)
Cons:
awkward to keep connected if not left on a desk\
::Edit::
After receiving a comment about my review of The Mophie Juice Pack Boost, I wanted to reiterate the cause and reasoning for my rating and review.
When I bought this product I wanted it as a means to charge my iPhone while I was on the go. I travel 100% for work so I don't have the traditional office time as many others do. Also I find it improbable that while I am in a stationary place I wouldn't be able to just plug my phone into a wall.
To me portable battery pack is something you take along with your and use while on the go not just as stand to use in your office that also happens to charge your phone.
That said I do think the device is worth the investment, it charges my phone and quickly. While I do wish that the connector would allow the iPhone to sit flush with the "Mophie boost" the device does its job. After a week of really using it, it is still a fine product but because of its limited portability while in use it is still 3 stars.
Thanks,
Kevin