Had my iphone6 for over 2 years, the latest iOS was giving me that dreaded "Your battery may need replaced" warning. Wouldn't keep a good charge, and sometimes would bounce between battery levels. Looked at new phone and then found this site. After seeing the instructions, tools and testimony i decided to go with this option.
With my phone the repair went very well. Used my MS Surface to follow along with the instruction steps as i went. The actual battery removal took some time as the sticky stuff gave me issues. After i got it out, going in reverse order was just as easy and going backwards with the directions.
Buy the plastic card. I had my little headlamp on (strap around head, 2 red LEDs and 5 white LEDS) which really helped to see. Take extra care around the battery tape removal. Mine almost came all out but still snapped, and ended up using the card. If you take your time and follow the directions stated, you'll be good.