I ordered a 3800mAh battery. It came with a custom back cover - all for $11.39, which included shipping from Hong Kong!
It works great. I am a light user - can go two days without a charge easy. Plus, I like the bump out on the back, as it makes it easier to grab the phone from my pocket and bring it out right-ride up, making it easier to locate buttons.
All in all, seems like a great deal.
Here's a couple of quick pics:
-