I bought two of these previously, looking to replace the 1800 mAh Duracell chargers I had. It's been a bit of a mixed experience...
On the one hand, the larger capacity has been a boon since I'd upgraded my phone to something a little more power-hungry than what I'd had and this handles things nicely.
On the other hand, the port is powered when the button is pressed - and it has this fun tendency to turn itself on and drain power as it sits in my pocket.
All in all I wouldn't buy again just because of the annoyance of not knowing if I really have a full charge handy given how easily it turns itself on and runs itself down.