When I was on a train in China couple months ago, I wished I had something like this. Although my Note 2 has a huge 3100 mAh battery, it still could not last long enough, because the phone (US SIM) always roam on 3G over there and signal comes and goes while a train is moving at 250 kpm.
I ordered 4, the maximum quantity for free 5-15 days shipping, total $7.92.
Not sure about its quality, but seems well designed and for this little money I have nothing to lose. According to its size, the cell should be really 2600 mAh. If its DC-DC converter is low efficiency, the actual output will be significantly reduced. For example, if DC-DC is only 50%, then the actual output will be only 1300 mAh. Even so, it is still worth the 2 bucks.
[update 12/18/2013] It's been almost a month and I still have not received any item or illegible tracking #. I opened a dispute with Alibaba, but the seller rejected it and offered partial refund ($6 out of $7.92). I requested again full refund. If the seller will play tricks again, I'll call my credit card company tomorrow to charge back.