Was wary because of iFixit reviews but so happy with my decision :))
I had my iPhone 6 diagnosed at the Apple Store and was told that my batter was consumed and needed to be repaired. It would cost me 79.99 + taxes to have them do it. I had already bought a battery and repair tools kit through iFixit for 28.99 (with shipping) and was weary about using their product because of reviews but decided to go through with it.
I found a youtube video online that made the process super simple and easy. The hardest part was getting the battery out of the iPhone metal casing because it is glued down with this double sided SUPER STICKY glue tape. The tools iFixit provided me with were exactly what I needed. After firmly prying that glued down battery from my phone for about 30 minutes there was no damage or extra problems caused.
Two thumbs up!
Know that the hardest part of the process is plying out that battery and that it takes a lot of patience, time and careful force. The youtube video I found recommends that you warm up the back of your phone (I did so with a blow dryer) for a minute or two to get the glue to come apart and make the prying of the battery easier. I did this over and over again (heating + careful prying with a tool) until I was finally able to get the battery out.