Not everyone needs a massive battery backup and not everyone wants the added weight you get from a battery case for your phone. The myCharge AmpPlus is a really great piece of gear if you occasionally run out of juice before the end of the day.
The battery provided about a full charge and a quarter when we tested it out. You get that power in a small device that measures 2 3/4" x 1 1/2" x 3/4" and weighs less than an iPhone 5. There's a built-in flashlight that's useful when rummaging around your bag. The battery costs $40 but it doesn't feel cheap. The plastic has a satin finish and the rubberized ends make it easy to grip when you're trying to fish it out of a bag.
The battery comes with a micro USB cable for charging. If your phone uses a micro USB charger, you can plug that cable into the other end and use it with your phone. Otherwise, plug whatever USB charge cable that works with your device into the other end of the battery.
If you want a lightweight battery, this is a great option.