I took my iPhone 6S into the Apple store today to get a new battery. After 2 1/2 years, the old battery was still at 84%, so they tried to talk me out of it. Uh, no thanks. New battery for $35, please.
I also brought my wife's phone in for a new battery as well. Surprisingly, her battery was 90%, even though her phone is a regular 6 and over 3 years old. Not sure if the new battery was worth it for her, as she's been having other issues with the phone and may very well replace it shortly anyway. Oh well, figured we may as well try. Hopefully she gets another 6 months out of it at least.