I received my New Trent iPhone 4 Power Rock case about 3 weeks ago and I am very happy with it.
Included is a micro usb cable and a screen protector which is a great bonus. The case is very durable and has a rubberized texture which gives it a high quality feel. It fits the iPhone snug and securely. On the lower backside there are four lights and a button. By pressing the button, the lights light up showing how much battery life remains. It is almost twice the thickness of the iPhone 4 and half an inch longer. Some may complain about the thickness, but considering the amount of extended battery life it provides, it is very much worth it. The size is also easy to overlook since I only plan on using it while traveling.
So how good is the battery life?
I've experimented with 2 ways of using the battery. One way is to leave the battery on at all times so that it runs in conjunction with the iPhone's own battery. The other is to turn it on when the iPhone 4 battery is running low. (By pressing and holding the button on the back, you can make the battery active or inactive.)
1. By leaving the battery on, I was able to use use the phone for 53 hours of normal use (for me) including listening to music via wifi and 3g streaming, texting and checking my mail.
2. By turning the battery on when the iPhone battery was at 4%, I was able to charge the phone back up to 76% giving me a total of about 48 hours of battery with similar use as before. (The phone probably would have reached almost 100% had I not used it during that time.)
Option #1 is my method of choice due to the better performance.
Overall, the New Trent iPhone 4 Power Rock case is a great value and keeps my iPhone going when I am unable to plug it in.