My nearly 3 year old iPhone 6 battery was so badly degraded that I could not use the phone for more than an hour or two without experiencing a random shutdown. When it wasn't shutting down, the battery would drain incredibly fast. It was almost unusable.
I tried to get a $30 replacement from Apple but six weeks after reserving a battery, they still have not called me in for my appointment. Oh well...
The repair went ok. The adhesive strips gave me the most trouble. The first one I tried to remove broke immediately but thankfully it broke off close enough to the battery that I was able to pull a little bit out with the tweezers and remove it without anymore issue.
The next close call came when I was attaching the new strips to the new battery. I initially put the wrong side of the strips on the battery and then had to remove/reapply them. They got a little bunched up and its pretty ugly but the battery fits in the case okay.
After that it was smooth sailing. Pretty easy repair.
When removing the adhesive strips, do not start pulling until you have hold of the white tape. The small black bit broke off very easily on mine and I almost lost the strip under the battery because of it.
When applying the new adhesive strips, remove the blue tabs to expose the tape and attach the battery to it while the tape is flat on a table. Don't touch or peel back the pink film until the last step.