To my knowledge, this was the first product for the iPhone 4 like this, available before christmas (or at least the only one in a decent price range). It does do what it says. Using a blackberry style cord it charges up your iPhone first and then your case. It can be used to connect to computer and your itunes account.
The design does add some extra weight, there are probably more expensive products available that will be slimmer. But its not too thick, it can be held easily in the hand and is robust if dropped. The top is thick enough that you may struggle to put in the old style headphones with right angle jacks.
The first charge was double the battery, after three months, it probably gives around 50% (maybe more) power. It is good if surfing the Web a lot.
Easy to put on and take off, I often don't bring the top of it out.
The best way to charge is to charge the iPhone separately to the battery, (the battery will then charge more quickly).
I probably wouldn't recommend this exact device as I think others will be better, but if you want a fairly cheap battery in a case then this is an option.