Klauss isn't overly impressed for the price:
Ha! The Onion Rings!
I've read a lot of bashing against the poor cheap Tamron in this forum for that very reason. I would lilke to know what those people have to say now.
Anyway, quoting the review:
The question of the day is, of course, whether this is "enough" compared to the impressive Tamron AF SP 24-70mm f/2.8 Di USD VC ? Well, we have some doubts here. We'd say that among the primary criteria the Canon lens has an edge in terms of contrast (at max. aperture), build quality and it has a slightly better bokeh. However, the Tamron lens is as good in the lower zoom range and provides a much better border quality at 70mm. Additionally it has a unique selling point - an image stabilizer. So unless you're heading into a war zone (thus requiring max. equipment quality) a premium of one grand (EUR) over the Tamron lens seems a little excessive.
I can only agree, and I add that it's actually utterly ridiculous.
They improved the build quality to a decency level but it's hard to praise Canon for that. They should be ashamed for the QC issues of the previous version, considering the price tag and the pro user target.
Looking at those graph it's hard to justify the hype for its sharpness either, and the bokeh is probably a tad worse than the previous version.
And the price? I would never pay more than twice as much as for the Tamron, which also has VC. By the way Tamron also offers 6 years of warranty and an excellent service. It's pure value.