A few months ago, I decided that, as we camp regularly, I could do with an external battery pack to top up my phone without needing a mains electricity supply. The Anker Astro Slim3 was well-reviewed on Amazon* and seemed reasonably priced for its capacity, so I ordered one. It arrived promptly and I can say that it is a very well-designed and useful device. It has a inbuilt micro-USB input which both serves as an output and covers up the input to make sure it doesn’t get filled with dust etc. It also has a female USB output, so you can plug in any USB chargers which came with the device you want to charge. It also came with a micro-USB to Apple 30 pin converter, also very handy. However, it developed a fault, so I contacted Anker customer services and they were very helpful and promptly sent a replacement, which arrived a couple of days later. I have had no problems with the second one, and am so impressed that I bought my brother a 3000mah Anker battery pack for Christmas!
Overall, I highly recommend the Anker Astro Slim3, as it is a very useful and reasonably-priced product which has enough capacity to recharge my S3 from 0-100% about 2.5 times!
*Unfortunately, the Anker Astro Slim3 is no longer available on Amazon because Anker have replaced it with a newer version.