They aren't the best, i have a few different ones.
They only charge my phone 1-2 times and take forever to recharge via solar. They need lots of direct sunlight to charge. If I put it on the outside of my pack while hunting it won't fully recharge if I'm in and out of the trees.
I prefer a std battery pack and separate charger-- I can use my bio lite stove to charge either the pack or my phone and cook or heat up my toes/fingers at the same time.