My iPhone 6 has been VERY sluggish for the past 6 months. Annoyingly slow, especially when typing. I came very close to pulling the trigger on a new iPhone. Just as I was complaining about how slow my iPhone was I read the news about Apple intentionally slowing down iPhones when batteries were "old". I thought AHA I was right! So I figured spending $30 was a lot more palatable than spending $1000 on a new iPhone, esp if that new iPhone would have the same issue within a year or so.
Repair went great. The instructions were very clear and the toolkit provided was awesome. The only snag was that one of the adhesive tape strips under the old battery did not come out cleanly, so I had to use a hair dryer to heat up the back of the phone and loosen the adhesive tape. Other than that it was easy peasy. It was really fun to open the iPhone and see the internals and how things are connected. I missed my calling! :)
Just be patient and follow the instructions. Make sure to organize the screws you take out since they are different lengths. Makes it easier when re-assembling.