I am so much on the fence between the Canon 24-70/2.8L II and the Tamron 24-70/2.8 VC. Would love to hear why you chose one over the other and if you are still happy with your decision. I am very aware of the physical differences between the two and the various test reports out there, but I am more interested in "how they feel and taste", if you know what I mean ... Comments?
I'm probably on the other side of the fence from most people here, as for me I'm not on a budget. I just chose the best lenses that are available.
I started out with the Canon 24-70mm f/2.8 II, this lens had been so hyped up that I was one of the first to line up to buy it. After all such an expensive lens had to be good.
It's worth mentioning that the 24-70mm f/2.8 II is the only apochromatic normal zoom lens made for full frame cameras. Apochromatic lenses are usually reserved for lenses you've heard a friend of a friend try at a show. They tend to cost $5,000+ and are made of pure moon rock's - I've heard. I hate color fringing and it's my least favorite image quality facet and so I jumped on the 24-70mm f/2.8 II like a kid in a candy store.
The 24-70mm II makes bad photos.
The problem with this lens is the bokeh, contrast and color. They're terrible. When they designed the lens, they messed up the correction for spherical aberration. This causes the bokeh to melt into it's surroundings and areas that are slightly out of focus to be mushy. You can notice a visible lack of contrast and color comparisons between this lenses bokeh and any other lens in this range.
Canon 24-70mm II:
Notice the mushiness?
I have never seen a lens make scenes look so bleached and ugly.
If you look at sample photos you can see this same effect.
Images from this lens look flat.
The 24-70mm mk I f/2.8 from Canon was worse as it has weird bokeh and the Nikon 24-70mm f/2.8 isn't much better (though it's the best out of the 3), but the Tamron 24-70mm VC really has this look that pops. It has more pop than the Canon 24-105mm, which is a lens that has a lot of pop.
The Tamron 24-70mm VC just has better color contrast and pop than any other normal zoom on the planet. Images from it simply look better.
Images from the Canon BORE me. They look pathetically lame and make me want to throw up. I'm a pro photo editor (I edited for Harper's Bazaar before I ever touched a DSLR) and I can manipulate color and contrast and character and texture extremely well so I can fix the flatness issue, but again the flatness is only in the slightly out of the focus to very out of focus areas. That means that to fix it you need to adjust these areas independently. The Tamron does not have this problem and so delivers good images without spot editing.
In the end it was easier to fix the Tamron's color fringing over the Canon's poor rendering of everything more than slightly out of focus, so I went with the Tamron.
If you have any doubts in what I'm saying take a look at this image:
Here we have a dog. Notice how his fur is perfectly contrasty and has nice sharp edges. Now notice the grass. Notice the dark areas of the grass. They are grey. The Canon 24-70mm f/2.8 II is a bad lens that makes bad photos. You should not buy it.
I bought the Tamron as a backup lens to use in emergency low light situations that required f/2.8 with VC and to stay in the bag 99% of the time and the Canon as my pride and joy. The Canon actually took such unusually bad photos that I had to stop, wait a second and think to myself "what in the world is wrong with this lens that is supposed to be amazing?". I wasn't even prepared to think that the Canon 24-70mm II took bad photos but they were so bad, I couldn't avoid noticing the problem, despite already making up my mind that I liked it. And the Tamron schooled it so badly that I actually preferred it after I had used a label maker to label it "For emergency low light use only".
Hope that helps, from somebody who's chose between the two regardless of price.