The Short Version: It’s worth exactly what you pay for it. If you really need a reliable, high-capacity battery case that’s worth the inconvenience of carrying a brick then I don’t recommend this case. Mine has sat unused on a shelf for months. I may just charge it up once a month for emergency use at home. It does a good job of maintaining a charge when unused for a long period of time but unfortunately I started having a problem with the case randomly stop charging and then restart, even when it was sitting on a flat surface. I suspect the dock connector is failing, which is similar to problems others have mentioned in product reviews for this device.
My iPhone 4 battery still works well but I have noticed that it doesn’t last quite as long as it used to, especially if I’m using it for power-hungry tasks such as playing a game or browsing the Web. I think I’ve treated the battery well by making it a habit to not leave it on the charger all night and instead only plug it in until it reaches 100%.
This week I started browsing iPhone battery cases. I perused the reviews for several, but I couldn’t make up my mind. The battery performance drop wasn’t enough to justify spending $60-$80.
At one point I jumped over to Monoprice.com and came across a backup battery case that had good reviews and only cost about $24. I went ahead and ordered one. It arrived today.