My daughter's phone battery would go from 50-60% to shutdown in minutes, especially outdoors in cold weather.
In addition to the repair kit, I bought an iSclack, an iOpener, and the magnetic project mat. The iSclack is worth its weight in gold. The magnetic project mat is worth ten times that. I didn't need the iOpener, as I was able to get the adhesive out from under the old battery using the primary technique in the guide. The hardest part was replacing the connector for the digitizer -- the instructions state that you mustn't press it in the middle. Out of the 30 minutes I spent on this repair from start to finish, ten of them were getting that connector back in place.
Also, the blue spudger that came with the repair kit is spectacularly well shaped and made for this project.
Plenty of light, reading glasses (if you're old like me!) and patience with fiddly bits. The instructions were spot on.