I have the Tamron 24-70 2.8 with IS, and I have to admit not been entirely happy with it, especially the sharpness when I AF it to a distant object at 24mm for landscape shots, specifically that should be at the hyperfocal distance. I probably need to spend time with MFA on my 6D to get it into better focus.
Also, if you forget to turn off IS on the Tamron when you are using it on a tripod, the results don't just sacrifice a little sharpness, they are quite blurry. The Tamron IS is very unforgiving in this respect. I've achieved excellent results even when I forget to turn the IS off for tripod shots on my Canon 70-200 2.8L IS II, but the Tammy makes you pay dearly for this gaffe, at least my copy of this lens does.
One silver lining is that I am getting much better at remembering to flip that IS switch off now. I used to forget about 25% of the time, since I tend to be mostly a handheld shooter. Now I forget maybe 5% of the time. Another thing is that the resulting blur is so bad that chimping on the camera reveals it pretty readily, so I usually catch it in time to correct it. With the Canon 70-200, sometimes I would fool myself because the images were still sharp -- in fact I usually don't really see a difference when I turn off the switch (I did notice a difference on my gen 1 of the same lens). Hopefully if Canon produces a 24-70 IS it will be on a par with the 24-70 II, and have IS that is forgiving of absent-minded photographers like me when using it on the sticks.