I've had mine just a couple of weeks and it works as advertised, you really do get twice the battery life. This means that if you charge it every night, you never actually use the iPhone battery, your iPhone runs only on the battery in the case. Must save you money in the long run as the iPhone battery is hardly ever used.
The phone looks fine in its case, it's obviously a personal thing whether you like it or not. It does take the edge off the beauty of the iPhone in my view, looking rather like something you made the iPhone wear as a punishment. But the extra bulk is a realistic amount of mass to carry around for a phone that does as much as the iPhone. Now you'll have two days battery life and can take the iPhone into the hills to run as your GPS. Insofar as it's a bit less cool-looking with the case on, it means I play with the iPhone less -- surely no bad thing.
The fit is great, easy to put on and take off. And as far as I can tell, the claim that the sound of the iPhone is improved is true. The case effectively increases the size of the speakers.
These Juice Pack Plus cases are heavily discounted. I've tried other cheap battery packs and they're rubbish. The idea of carrying an extra power pack, that will work but you won't easily be able to use the phone while you're charging it. It's all too easy for the connection to break if you're carrying a phone in a pocket or bag connected to an external battery. Then you open your bag and find your phone hasn't charged at all. The Juice Pack Plus will charge it on the move, or better still, run the phone off the Juice Pack Plus battery, at the cost of a little higher power consumption.