My battery was routinely getting to around 35% and then immediately depleting to 3-5% left. After several calls to Apple and a trip to the Apple store, where I even showed a team member there what is was doing (it did it right in front of him), they insisted through their 'onboard' diagnostic that nothing was wrong. Look, I have 3 engineering degrees and an MBA. I know just a little bit about how batteries and electronics work, and this of course was a load of crap. So I ordered a new battery from iFixit to, well, Fixit!
The repair went well. I ordered the battery with the supplies plus the iOpener kit as well, and I'm glad I did. The Philips screwdriver in the basic battery kit for me did not work well (the pentalobe one worked fine). The one in the iOpener kit was much better. When I went to pull out the battery adhesive strips, one came out just fine, the other snapped off after getting out about half of it. So out came the iOpener.
My only complaint overall is that the iOpener KIT is not for the Apple devices, as it doesn't have a pentalobe bit, and there is some overlap in tools. If you ask me, iFixit should sell 3 versions: battery only, battery + basic tools (the one they have now), and an iOpener kit that has a pentalobe bit in it as well so that it is the basic tools version + basically. So you have to order both the battery kit and the iopener kit to get everything possible you need.
Beyond that, go slow, have good lightening, place the screws in a way you can remember how to put them back, and press pretty firmly down on the connectors as you hook them back up, and you should be fine