I bought this after looking at all the other battery cases for the S III. At the time it was offered for $40.
This case will charge the battery, but there's no real way to verify the capacity. I tested it by letting my phone battery drop to 39% and then activated the battery case which was fully charged. The first thing I notices is that the battery case gets *really* hot. The external micro USB port was almost too hot to touch while the case was activated. I seriously will not leave my phone unattended while it's charging. The phone was charged from 39% to 85% before the battery case was exhausted. It did this in about two hours.
Given that the stock battery is 2100 mAh, I expected the battery case's 3200 mAh battery to double the life of the phone at the very least, but in reality, it only boosts the capacity by 46%. If the battery in the case is actually 3200 mAh, I suspect most of the energy is going into heating the case up.
It's better than nothing and it will basically give me a litte boost until a higher quality solution is made available. That's the best I can say about it.