I love my iPhone 4S, but I am one of many who doesn't get great battery life from the phone. Apple has done some firmware updates that 'addressed' the problem, but I think battery life on my phone is still sub-par.
I've looked at a few battery cases, but many were pretty clunky and didn't seem worth the hassle. A friend tipped me to the Boost Case, and I found it on Amazon and gave it a try. The case I ordered fits the iPhone 4S and iPhone 4, and is made of a hard rubber nonslip surface. You charge the case with a micro USB cable (included). The flat built-in battery puts out 1700 mAh. I plugged it in overnight, and blue LEDs on the back of the case confirmed it had a full charge.
I slipped the iPhone into the 2 piece case, and started my day. Every time I looked at the screen, the battery reading was 100%. It was 100% through the morning, through lunch, all afternoon, and well into the evening when the iPhone started running on its own battery and I saw 92% about 10PM. The phone got normal use through the day. Lot's of email, some snapped photos which were forwarded to others, even part of a Netflix movie and the usual phone calls. I tried the same routine the next day with the same results.
When I first got the case the iPhone reported that 'charging was not supported with this accessory', but the iPhone was indeed charging. After the first try I never saw the message again. I tested photos with the case on to make sure the case did not interfere with the camera lens. Pictures looked fine with no obstruction visible. Some users have reported shooting with flash can leave a bit of a haze at the lower right of the image. I made a few tests and the images looked fine, but your experience may vary.
The Boost Case comes in a variety of colors, some models have a removable battery pack, others have it integrated. The one I tried uses an integrated battery and is only US $47.50 at Amazon. That's a pretty low price to completely solve the vexing problem of iPhone battery life. There are plenty of cases out there that provide battery extending functions. The Boost Case works very well and is worth a look if your iPhone battery life is less than stellar. Feel free to share your experiences with the Boost Case or any others that you have tried.