I had a chance to test these lenses last Saturday. The body I used was the canon 5d m2. Here are my first impressions. Both lenses are on par with the build quality, Canon may have a slight edge(better weather sealing), but the Tamron felt better in my hands. Sharpness: The Tamron is very very sharp! The Canon is sharp but Tamron has the lead. Contrast: There must have been something wrong with the Canon i tested, or Canon has done this intentionally so people can adjust the contrast later on. Resolution and detail: The Canon is the winner with near twice the resolving power! Auto focus: Tamron is impressive, not super fast, but nails the focus every time. The Canon, was the focus king though, with one exception, the focus was all over the place! I needed to refocus three times on most of the shots for the focus to lock on the subject. Overall, I would buy the Tamron. More than half price less, + VC, oh and i forgot to say, what happened with Canon's bokeh?? All I saw was gausian blur, and there was nothing wrong with the tamron, i've read people complaining about its bokeh... Actually it was one of the best bokehs i've seen on a 24-70mm range lens. I'm getting mine on the 5th next month
Hope this helps people decide on which lens to buy, if they had any doubt about the Tamron -Go for it!
P.S. I have only a few sample images (same theme) with both lenses i'll upload them latter on today.