Replaced battery, improved performance; some iFixit part disappointment
I chose to replace my iPhone 6 battery after reports of Apple artificially throttling the performance of older iPhones as their batteries aged. I'd seriously considered purchasing a new phone, assuming (like many others) simply that the newer iOS was too feature-rich for my old phone.
Repair was simple, just like the instructions. I did NOT go to the trouble of removing the front panel (glass and digitizer), based upon what others have written in comments about difficulty in re-attaching the cables. I used a spudger to old the top panel back, but you could also rubber-band it to something to hold it open. I estimate this saved me 15-20 minutes of time.
This repair is easy if you have solid experience opening and repairing electronics. The iFixit toolkit was helpful, but absolutely not necessary. The only really helpful part was the Pentalobe bit, but an experienced/well-equipped PC tech will already have one of those in a precision bitdriver set. The spudger was also helpful. If I had it to do again I would only purchase the battery and not the toolkit.
The "liberator" Phillips screws that were included to replace the Pentalobe screws did NOT fit my phone -- they are too long.
My only complaint is that the iFixit replacement battery's cable has a very sharp/firm edge that doesn't fold properly against the battery once installed. This causes a pressure point on the LCD when the phone is in use. I even re-opened my phone and tried to fold it against the battery, but it takes a lot of force and I was concerned about breaking the battery wires inside the connector.
FYI - The battery replacement at least doubled my phone's performance. Totally worth it. DO perform the "hard reset" at the end of the iFixit instructions.