Yes, it adds a lot width to the phone making it virtually unpocketable. It comes with a new back. It should already fit the ATT version because there are no form factor changes from the international version. Everything is the same: camera, flash, speaker, etc.
However there is a better solution in my opinion. New Trent and Superb Choice make the best external power bricks, and can be found a Amazon. I have the 11,000 mAh, and my wife the 5,000. The 5K is perfect.