My battery would drop from about 30% to essentially shutting off. Phone is a little over 2 years old.
Everything went according to the instructions, including the tab on the battery tearing off. I was able to get the battery out by gently levering between the case and the battery. It made a sound like pulling tape off fabric then it came free. Battery connection cable from the ifixit replacement part stuck up a little so i bent it down over the battery after I reassembled all the innards.
I dropped one of the tiny screws into carpet. That was fun. I used buckyballs to find it.
Overall it went ok imo.
A magnifier is a handy thing to have around, as is bright lighting.