I just tried the Sigma 24-70mm today, verdict, did not like it.
Soft wide open, at all apertures. Although some have reported good copies.
Very awkward placement of zoom ring... I kept turning the focus ring thinking it was the zoom ring -.-... The focus ring is in the middle of the lens, and the zoom ring is actually very close to the mount. As a result, the focus ring turning during AF tends to get in the way of holding the lens properly.
Looking to try the Tamron, but I understand that lens has bad AF.