Original battery was getting worse and worse. Something had to be done!
Over the past several months, I started noticing a significant decrease in battery life on my iPhone 6. It used to go two or even three days without a charge, but it got to the point where it wouldn't even get through one day and started just shutting off even with 26% or so remaining.
Overall, compared to the other repairs I've done, this was very easy. The display was very easy to open up, and the old battery came out without a problem.
I did run into a couple of minor issues, one of which I think should be noted in the instructions (or perhaps it was and I missed it). When installing the new battery, I did not realize that the connector and cable needed to be bent a certain way to match the original one BEFORE setting the battery in place. Because of this, when I initially placed the battery, part of the cable was sticking out in such a way that I wouldn't have been able to install the display back into place properly. Luckily I had enough room to bend the connector to match the original one and was able to make it work, though it was more difficult doing this with the battery already stuck to the phone. The second issue I had was after putting everything back together, there was one small vertical section of the digitizer that did not work. The solution to this was to simply re-seat the display connectors. After that everything worked perfectly, and my battery now lasts two full days again!