I got four of them for my whole family, we're all very satisfied. It takes a while to charge the Astro Slim3, but the charge lasts for a while, recently I've fully charged Lumia 720 (2000mAh battery), Xperia arc s (around 1500mAh) and more then half of Nexus 7 (4325mAh battery) until the charge drained. I then left it overnight to recharge, and it was fully charged in the morning.
I really like the integrated cable, so you don't have to carry any additional cables with you (unless you want to charge two devices at once, which Astro Slim3 is capable of).
The only downsides I've encoutered so far: the integrated cable connector won't fit if you have a case on the phone but this all depends on where USB port is and how thick the case is (it fits the Lumia with case very nicely, but not xperia and nexus 7). There are no issues when plugged in with no case on the device.
Another downside is that the provided USB cable is charge only, it won't work for transfering data from the computer, but since this is a battery and the provided cable is meant for charging only, this is only a slight inconvenience rather then a con.
The battery itself is nice and slim, it looks nice, it fits nicely in your pocket.
I recommend this product.