MacBook Air 13" Mid 2012 Battery
Update -- after about a month with the A-Stock battery, I got the "Service Battery" warning, but it did not stay -- disappeared after a while. Strange behavior. I would not recommend buying anything but a new battery based on this experience.
Very easy. I bought the "A-Stock" battery. It came at a max charge of 6300mAh (max of 6700) -- decent. But within 2 weeks dropped down to a max of around 4,000 mAh. I'm thinking that I should have spent the extra $10 and bought the new battery. (I am using the Coconut Battery App.)