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I don't have a problem with Tamron, but their AF consistency is not as reliable as Canon, and several of their lenses have had problems with outer AF points especially. If AF consistency, sharpness, colors and the like matter to you, the verdict is still out. I'm sure it will be a fine lens, but then, so is the Sigma. Only time will tell, but there are plenty of people who will stick with the Canon. Clearly is is longer and likely has better stabilization, and if that is your only criteria, the new lens looks a winner.
I have no AF problems with my Tamron 70-200 VC and it is just as reliable as the Canon 70-200 II that I had at the same time. The Canon in low light was a fraction faster to lock (only VERY slightly) but Tamron's AF is just as reliable. As for longevity, Tamron comes with a 6 year warranty.
I agree that the verdict is still out. There are only a handful of opinions of actual use out there. We'll know a whole lot more in 30 days or so. My comments are based on what we've heard so far. If what we've heard so far turns out to be true then that would be huge!