I was skeptical at first, but after using this for a couple weeks, it turned out to be a solid device. I have heard many people opine the device's design and lack of usual Apple flair, and I have to agree, the design is somewhat obtuse. However, from a practical standpoint, I have come to appreciate it. Before I tried this device, I used Mophie's popular Juicepack Air. However, I did not care for the extra bulk the Mophie added. When I started using this case from Apple, I immediately preferred its ergonomics. Let me explain. In the middle of the device, you can definitely see that it matches the thickness of the Mophie Juicepack Air. However, the "ungainly" hump on Apple's case is nice from an ergonomic standpoint, as the battery bulge and extra thickness does not cover the entire back of the device. Instead, the top, bottom and edges of the back are pretty close to the thickness of a regular rubber case. So at least for me, when holding my phone with the Apple case in my hand, I preferred how easy it was to grip the bottom and sides compared to the all around thickness of the Mophie unit. As far as weight, I found the Mophie and this case to feel similar in the hand as far as heft, and quick acclimated to both.
As for performance, this case definitely differs from the Mophie case mentioned above in a few key areas. First, overall capacity is pretty similar, with both cases extending your phone almost a full charge. Whereas the Mophie case can be turned on and off at will with its power button, this Apple case works automatically. So essentially, after plugging in overnight, when I take my phone off the charger, the case will discharge slowly, keeping the phone at 100%. Once the case discharges, you still have 100% phone battery left that then is used by the phone. The only way to know the battery charge status of the case while in use is to look on your iPhone's battery status indicator. However, if you are charging the case by itself, without the phone inserted, there is a status indicator on the inside of the case. So if you like having the option to decide when the case turns on and off, this is not the case for you. However, if you prefer just to unplug and go, without worrying about having to press a separate power button for your case, this one is for you!
A couple other notes. In my use so far, I have not felt the Apple case get overly warm at all while charging the phone. When using the Mophie case, there was a noticeable increase in warmth, though never dangerous or concerning. Just something to think about. Additionally, this case uses the same lightning cable as the iPhone and has pass through charging, so it will charge your phone first. If you go with a battery pack other than Apple's, you will most likely need to charge with a micro USB cable, so something to keep in mind.
In the end, as I mentioned at the start, I prefer the ease of use and ergonomics of this device over the Mophie Juicepack Air. I love the idea of never having to run out of battery during the day, no matter how much I use my phone. For me, this case was definitely worth the investment.