The thickness of the battery is double of the original, i.e., 10.8 mm vs. 5.4mm. Therefore, its actual capacity is more likely to be 2x of the original, 5200 mAh. Still not bad, especially for the money.
The new weight is still very acceptable at just 6 oz, well the new back door does add bulkiness to the very slim body of S4.
Some friends told me that these cheap batteries do not hold their charges after 1 month or 2, let's see. All I can say now is so far so good and for just $10 I do not worry much. There are some 7200 mAh ones on ebay for $30. The extra $20 does not worth the higher labeled capacity, but I wonder if their performance lasts longer? $30 is kind of too risky to try.
[update 04/19/2014] The Anker 5200 mAh battery gets very good reviews on Amazon, cost only $10 more than above no-brand ones. Anker even makes a whopping 7800 mAh one, if that's interesting to you.
[update 04/19/2014] The Anker is good.