Tamron is sharper than Canon, it is F/2.8 with image stabilisation comparing to Canon F/4 (you can shoot with Tamron in much darker places), autofocus speed is almost the same, vignetting at F/4 is much better in Tamron, Tamron is less expensive.
My conclusion would be "Shame on you, Canon"! It seems that third party manufacturers started to produce much better quality production, which is much cheaper. This particular lens is better than Canon 24-70 L 2.8 Mark I and slightly worse than Mark II (which is much pricier). I would say that Canon should really revise its pricing strategy and focus more on quality in order to compete with Tamron and Sigma.
I really like L class lenses, however, I hate to be screwed! I bought Tamron SP 24-70 2.8 instead of Canon 24-70 2.8 L II because of price and lack of image stabilisation in Canon. I am very happy with Tamron!