My battery was not holding a charge very well. I was one of the first people I knew to get the iPhone 6 and my phone has been through a lot over the two plus years I've had it. My phone would just die randomly around 39%. Something had to be done.
EASY EASY EASY! Firstly, iFixit makes everything extremely easy. Parts kits, in depth video tutorials, perfect pricing and fast shipping. The whole install took a little under 10 minutes after watching two of their videos. I feel confident enough to do other people's phones and charge them a fee.
Advice. Find a big open table with a grippy mat to lay down. I emptied the tool kit and lined up all the tools near the phone. I worked one section at a time, placing the screws I removed in the empty tool slots to help keep track of what screws came from what holes. Take your time and don't lose any of the screws. I wish I had more advice on how to get the microscopic screws into microscopic holes, but just use a lot of patience. That took almost as long as removing the battery tape, but still the whole thing was 10 minutes. ALSO Follow the battery conditioning instructions. My phone has lasted a day and a half without needing charge, where my old battery would have only lasted a few hours.