I ordered the iPhone 6 / 6s Battery Case, Anker Ultra Slim Extended Battery Case for iPhone 6 / 6s (4.7 inch) with 2850mAh Capacity / 120% Extra Battery [Apple MFi Certified] (Black) a couple months ago. It was delivered promptly and I installed it in mere minutes. The case has worked great, protecting my phone and extending the battery life tremendously. I can easily go for at least 2 days without having to worry about recharging. The battery backup system works great and is easy to use.
Pros: Works great, extends the battery operation to more than double the standard battery time.
Cons: Not really a con, just something I observed that could be considered a con. When the unit is charging from the backup battery, it can get quite warm when in the pants pocket. Not hot enough to burn but noticeably warm enough to be uncomfortable. It’s just something to be aware of.
Overall the case/ battery backup is an excellent option. I have tried others but the quality of the Anker is superior to any others that I tried.
Anker seems to always deliver high quality products at reasonable prices, you can’t go wrong with an Anker product!