I just bought this
case off of Amazon and stuck it on my AT&T One X+. I bought the last "used" one for only $25, but as far as I can tell, it's in brand new condition which is awesome. It fits the phone perfectly, though it's definitely chunky, but less so though than I though it would be. And I gotta say, I'm liking the heftier feel to the phone. The shell is really smooth and feels great in the hand. The main reason I went for this case over a few others was that it actually makes use of the pogo pins on the back of the phone, so there isn't a stupid USB cable sticking out of the side like this
case. The battery in the case is hot swappable, so you can put in any similar sized battery to charge your phone if the one inside dies. It's the best of both worlds when it comes to a phone!
The battery that comes with it is rated at 1750 maH, which is almost 75% of the capacity of the internal battery, so it has added many hours of battery life to my phone in day to day use. I'm still gonna need a couple more days to test it out, but if anyone has any questions about the case, let me know.