Positives:
The battery pack has 4 status LEDs and an On / Off switch, it invites therefore not always the battery, but only when needed.
You can download the iPhone in the battery pack, load both in parallel, but with priority for the iPhone.
After today was my iPhone 4S (iOS5 on 29% down, I charged by the battery pack. In the very beginning it has been loaded by 40%, then in 30min. To about 90%. Then, the battery pack switched off and the Status LED blinks (So what I'm on about 170% total capacity - but because the battery pack is new, it is perhaps a little better when the battery was incorporated.