Not the most powerful for that price but a good choice nonetheless

Disadvantages:
1 / The battery is only 2600mAh where, for the same price, we find that offer almost 4 times more autonomy. Is it important? It all depends on what you think recharge with. Iphone 5 is a 1440mAh (so you can reload 1.5 times), a Samsung S5 in fact in 2100 (thus a little more than a single charge), ...
2 / No diode for displaying the remaining charge.
3 / slightly larger models but are found, too, with tube-shaped for less.

Benefits:
1 / Clearly very very small model. I have in the neighborhood of this size but they are still generally larger than 2 cm good. This round is super easily into the pocket. Hardly bigger than a lipstick tube.
2 / material that gives a good feeling of solidity. A kind of thick metal.
3 / The big plus in my opinion it's the wiring. Often on powerbanks, provided cables are very fragile. Here not only the cable is not fragile at all but in addition, it offers the 3 most common on cellphones tips: Iphone 3 to 6 and mini-usb found on 99% of Android devices and Windows Phone. The finish is beautiful and the cable is flexible while being thick. I think it will last much longer than any other I had to test recent months.
4 / impressive load speed. I compared with other powerbanks in my possession and it's one of the fastest. I guess the quality of the cable has to play too.
5 / The metal tube can be removed to keep only parts of connectors and battery. If you are a handyman and want a USB power pack you will be able to easily retrieve items for use in another media. Example? I did it with a USB fan that I fed with a disassembled Powerbank (link to the fan used: ;camp=1642&creative=19458&creativeASIN=B00J0BXWOQ&linkCode=as2&tag=leblodeman-21&linkId=3EEIOLF2XVEOSKDS)

In summary:
A priority if you want an external battery small and very good finish. You will recharge your phone 1-2 times and the maximum will be very fast.