I got one for my 6 last time and it was kind of a pain until it broke down completely about a month ago. The frame that clips the phone onto the battery pack is an awkward fit and made of brittle plastic, so it just snaps after a while. I continued to use the battery pack for a while, but it eventually gave up the ghost.
I have the lime green for my 6S. I like that I can find the phone easily and that it extends the battery life. I don’t like that it’s slick so the phone slides out of my hand and out of my pocket. I don’t like that it offers no cushioning protection whatsoever. My screen now has a top-to-bottom crack despite its glass screen protector.