I got my Powerskin this week and so far got the chance to use it for two work days. My work day consists of an 1:20 minutes of commuting time in the morning both on bus and subway, listening to podcasts, surfing the web, etc. And the same coming home during the evening.
During the day, usage on voice is minimal but push email via Exchange and other push notification are going.
Like most battery case solutions, when you turn the powerskin on, it puts the iPhone into a constant charging state using up all of it's own 1200mah then leaves the iPhone on it's own battery.
The case is indeed does feel slimmer than Mophie but it's based on memory each time I went to the Apple store and held the Mophie in my hands (with my device in it). I think for comparison purposes, I might actually buy the mophie, use it for comparison, then return it to Apple.
Another thing...not one issue regarding phone reception. This is in NYC and the surrounding 5 boroughs so take that for what it's worth.
If I have any complaints, it's regarding the mini-usb charging port on the device. Mili includes an attached cover for the port that you need to move out of the way to plug in the mini-usb plug. It can be a pia sometimes but it is what it is.
Overall, a very nice product. I plan to do a full pic and review as soon as I pick up a mophie to compare it to.