After 3 years of heavy use my original battery was loosing capacity significantly. The process process of battery change in Apple Authorised Service was very complicated - two visits (one for diagnostics and second for battery change) and leaving my laptop there for one week was not acceptable.
All went very ok, from moment of order to tighting last screw it takes 4 days. Total amount of work was less than 10 minutes.
Do it, it is very easy and cost / time effecitive process. Mainly if you can't be without computer or let it in the service due to security reasons (like me).
T5 Torx Screwdriver
P5 Pentalobe Screwdriver Retina MacBook Pro and Air
MacBook Air 13" (Mid 2011/Mid 2012) Battery