I found to be one of the easiest repairs, just one step above replacing batteries in a remote control. $15 and it's like new. Just be sure that you have the right tool--T5 is Torx 5, there is no substitute. Also, I pealed off the adhesive from the old battery and put on the new--without that it really did flop around inside.

Bought the OWC recommended drive from fixit (for 2012 Macbook air) and it was pre-formatted—so was able to move right on to restoring from time machine backup — worked great!