I use it mainly for my iPhone. This item charges the iPhone as quickly as it does if I plugged the iPhone into the Apple A/C adapter. If this battery is full, it can charge my iPhone from 20% to 100% without issue, but definitely only once before I have to recharge this battery given its moderate 2600mAh capacity. So for basic charging it works fine.
Two big gripes I have though:
1) If I use this battery to charge something that doesn't require much current, like my Fitbit, it will turn off on its own within a minute because the Fitbit is not drawing enough current to keep this battery on. I suspect other devices that don't require very much current to charge will also run into this problem. Flip side though, this is proof that it does turn off automatically to save power.
2) There is no indication of how much battery life is left. Even if it had 2 or 3 extra LEDs to give a crude estimate would have been good. This battery doesn't tell you if it's running out juice until the LED turns red, which means you're just about to run out of juice.
I won this in a raffle so I can't complain, but if I had to pay for it, I wouldn't pay more than $5-$8 for it. In a pinch though, this thing would work fine and is fairly compact to carry around. Would recommend if it were cheaper, like on sale or something.