This little device acts like a protective case for my Samsung Galaxy S3, but when my battery dies on my phone (which is often) you turn on the case, and it gives you another FULL battery life to keep you going until you can get home to charge your phone!
Had to have it, bought it on amazon.ca (or amazon.com) for only $14.00 and have loved having it!
The only bad thing I can say is that it IS a tiny bit chunky, but when you need that extra battery life I dont think it'll be a deal breaker.