If you're looking for a slim battery case to charge your iPhone on the go, but you don't want to spend more than $100, the Incipio Offgrid Express is a good $80 option (or £60 and approximately AU$110). The accessory offers a meaty 3,000mAh battery and it can double your phone's battery life for a good deal less than Mophie's $120 Juice Pack Plus.
Though its battery reserves aren't as high as the Juice Pack Plus (the Mophie sports 3,300mAh battery) and its design isn't as clean cut, Incipio's case does essentially the same thing and works just as well.
To use the Offgrid Express, plug the handset into the battery charger and snap the bumper around the phone's edges to attach both pieces together. For the first few times, I had a hard time snapping the bumper into place all the way since there always seemed to be a gap at the seams. But as I kept fastening the case together throughout the day, I eventually got the hang of it.
On the bottom right edge is a button that turns on the indicator lights so you know how much charge is left. You can also long press the button to turn the charger on and off. The case uses Micro-USB charge, which is common for iPhone battery cases. Because of this, you'll need two different cords to charge your case and iPhone (the latter uses a proprietary eight-pin Lightning connection). If this is a big deal for you, Apple has its own Smart Battery Case that uses the eight-pin port too.
During my time with the case, I saw that it did attract some fingerprint smudges, but they're much easier to wipe off compared to Mophie's case. The Offgrid Express also adds extra bulk to your device, especially at the bottom. Because your headphones may not fit through the case and into the handset's headphone jack, Incipio included a headphone port extender. In addition, a small rectangle on the left of the case is cut out to allow users access to the phone's ring/silent switch. Given the overall thickness of the case, that means you'll need to dig your fingers a bit deeper to toggle this on and off.