I have one of these, very happy with it. Provides at least 2, sometimes 3 charges for my iPhone 6s. It’s as strong as it was when new about 6 months ago. It takes a long time to charge, about 6-8 hours, depending on how far you run it down.
Only complaint would be that the blue charge indicator lights on the battery stay on while charging and they’re very bright - so you need to cover it if you’re charging it at night.