I have the new Tamron 70-200mm 2.8 for Canon mount in my hands for tonight and tomorrow. I gave it a try and decided to see how it compares to my Canon 70-200mm 2.8 IS.
In short, the Tamron is much sharper (in my opinion) and can focus much closer. It does have more CA than the Canon and is much slower to focus as well as very loud. It feels very well made and weighs a little less than the Canon. I don't like the focus clutch at all.
The Tamron looks like a winner so far. I'm sure it's what Pentax owners have been asking for over the past four years. I'm willing to bet it can out perform even the Pentax 80-200mm from what I've seen so far.
Here are two images for a sample test. Both were shot at f4 hand held and are 100% crops with no PP or size adjustments at all. The top one is the Canon shot at 1/640 at 165mm with IS on. The second was the Tamron shot at 1/1600 at 87mm. I know this is not a proper or even test, but I had very little time as it was sunset. I will give them both a more fair test tomorrow and post the results here.