Ex-Apple Genius still fixin' thanks to iFixit!
Sold my OG Retina MacBook Pro to my best friend and shortly after the battery started acting up, "Service Now" message
The repair went really well. I left Apple after 6 years of repairing their devices and iFixit has always come through with quality parts, they're the only source I trust outside of Apple Stores. The packaging for the battery was good, but the top middle portion below the battery board was a little bent. The battery went in fine and the cells were not damaged so it's all good. I'll double check the fitment and condition in a week.
Be extremely patient with the adhesive removal and new fitment process. Follow the repair guide verbatim and it will go smoothly. Take your time and clean ALL the adhesive off the casing before putting the new battery in. The one problem I had was the repair guide says to check fitment before removing the plastic at the bottom; I didn't do that so I had to be very careful while fitting the new battery. I screwed in the top cells first and placed the adhesive'd cells off to the side along the corners (as pictured in the guide's battery removal section) while making sure the battery cables(?) were routed properly. All in all, this was a great experience and it shows me that I can definitely help folks extend their Retina MacBooks life without having to go to Apple to pay twice the amount of money and wait for a more lengthy repair. Also, if this is the first time you're doing this repair ABSOLUTELY get the part with the kit, since it includes the adhesive remover, that with the extra backup tools are totally worth the extra $10