Tested the Tamron 3 weeks ago and the Canon one week ago. Tamron images were nothing special, ok sharp, but with pretty bad corners - mediocre sample??? Canon images were so incredibly sharp with great colour I knew there is no turning back and going for less. Combined with the 100IS Macro I can cover 95% of my needs without worries. In fact, with the Canon 24-70II for the first time I had quite a bit of moiré in the 6D images. I didn´t even know the 6D could be so sharp... Plus, after having used a Olympus 14-35/2 for years, I always wanted a standard zoom in the same league. Compared to this legendary lens, the Canon seems to be very very reasonably priced at the moment.
In Europe the Tamron is 800.-, the Canon is 1650.- (after 250.- cashback). I´m sure I´d have burnt around 250.- the moment I bought the Tamron - with the Canon there´d be very little loss in reselling, especially as the cashback has ended.
A friend of mine once said - about buying his bread and butter broadcast lens - there is the one bad moment when I pay, but there is a smile on my face every day after that.