I had a so called 'genuine' one from eBay
Turned out to be a refurbed OE battery and held charge for less than an hour
In the end, lumped the £85 from apple and bought a genuine battery - 6 hours charge instead of under an hour!
The copy batteries on ebay, especially for apple, are really poor quality due to the specific type of Lithium Polymer technology apple employ in the battery cells, that other manufacturers struggle to get right.
If you know someone with Uni/Work discount, you can get up to 18% off that price
Also to add... Depending on how old that new battery is, Apple may well replace it under warranty FOC