I won't repeat what everyone has said about how well it extends the battery, but I will comment on one thing that people have left out: how this case affects the camera.
When taking a picture with the flash, there is a distinct white "smoke" that covers most of your picture that is caused by the case being too close to the flash. Take the case off and take the exact same picture and this smoke is gone. This is extremely disappointing because the quality of the camera was one of the main reasons that I purchased the phone. I'm still not sure if I'll be returning it yet but I'm definitely considering it. Battery life is important to me but it isn't worth diminishing a core strength of the product because of it.
At first I didn't realize it because I pretty much instantly put the case on the phone, but when comparing pictures to my girlfriend's iPhone I noticed hers were distinctly better.
I'm not really sure if any other cases out there are any better because it seems like they are all pretty close to the same shape/design as the BOOST case. I'm pretty disappointed and might just be switching back to a regular case that isn't going to make all my photos look bad.