I've seen a lot of talk about AFMA in this thread and not a whole lot of mention about how the camera can be the main cause of it.
This lens needed +8 on my camera, which could mean that my camera is perfect and the lens needs +8 or it could mean that my camera is -8 by default and the lens is perfect but needs +8 to work with my camera.
So far this is an AMAZING lens (and I'll be posting some of my own photos soon!) and is amazing on your cameras if your AFMA falls anywhere between -20 or +20. A high or low number does not indicate a bad copy.
To solidify my point: I bought two 5Dmk3's recently for my business and all of our lenses needed nearly the same adjustment on both cameras, which are very close in serial number. All these lenses needed around -8 from their value on the 5Dmk3 when put on my old 50D, indicating that the 50D was -8 compared to the mk3's.
So if you can get your lens sharp with any number, be ecstatic
If you need more than +20 or less than -20, that's when you need to send in your lens and probably your camera too...