You have to really want that wireless charging to see the value here. Compared to something like the Anker Extended Battery Case, the 1,560mAh pack in the iPhone 6/6s Juice Pack Wireless dies sooner, even though it costs more. It’s a similar story with the 2,420mAh battery in the 6/6s Plus model.
And per usual, the case itself connects through microUSB, instead of the Lightning cable that came with your iPhone. (Only Apple’s case avoids this annoyance.)
Still, provided you keep it charged, it should still be enough to last you through your next train ride or Saturday night out.
In my own testing — in which I kept the screen on, at 50% brightness, with nothing else activated — I was able to get a dead iPhone 6s Plus up to 62% over the course of 100 minutes. Keep the screen off, and it’ll get close to a complete refill before passing out. The case itself takes about two hours to refill. All of that’s okay, but nothing more.