I just received mine yesterday and I have to say this lens is freaking amazing. Weight is about the same as the Canon 24-70 , Solid built.
To me the quality is pretty much as close as Canon. Focusing is super quiet. Before I purchase this I was debating should I get the Canon 24-70 or the new 24-70 II ? But when I thought about
the VC from the Tamron, you can't beat it. You spent over $2000 for Canon and still don't have IS. This VC from the Tamron is bad ass, it helps a lot in low shutter speed. You still can get sharp pictures.
For $1299, this is well worth it. I bought it from B&H. I also went to WPPI in Vegas back in Feb. At the Tamron booth, they didn't have the lens for demonstration, but tons of the people were asking about it and
they show the post production quality of that lens. Since then, I already have that in mind, this probably my alternative. Since the Canon 24-70 cost too much (without IS).
Trust me, you won't be disappointed :-)