There are probably bad copies of this lens out there and I was unlucky enough to get two lens in a row that are bad.
First one has back-focus problem. Shooting test against 18-55mm kitlens, 50mm f/1.8 and 28-135mm IS confirms the problem. Return to BH photo for exchange.
Second lens is even worse. The electrical contact on the lens has problem. It won't communicate with the camera. When I first mount the lens onto the camera, it works fine. After a new shoot, it stops working. The camera can't autofocus it and can't shoot. The ERR 99 message on the camera confirms it is a bad copy.
So, I am returning it too.
This makes me really appreciate my 28-135 IS lens and 50mm f/1.8. I really want to love 24-70L, but, it disappoints me twice already.