I have one. I did have some issues with my Juice Pack. Some instances it won't activate when the switch is moved to the "On" position. Even when pressing the status button, none of the LEDs light-up. At that point I thought it was dead, so after plugging it in for 1/2 second, it came back to life and showed that it was fully charged? Another time when I was charging my phone with the Juice Pack, an error message displayed stating that I was using an unapproved device as a power source and that it was recommended to disconnect it. I didn't figure that one out until I saw that the charging indicator light on my One wasn't lit. I do think I might just have a defective unit since the behavior is inconsistent. However, when it's working correctly, I'm more than satisfied. It meets my main criteria of having additional power supplied in an uninterrupted manner.
One more thing I found out is that it's mostly designed to charge the phone from 20% to 80%. I don't think it can do 0% to 100%.