Either there is a major difference in IQ and quality control, or the reviewing website I most often use (you know the one) is somehow biased toward Canon (and maybe Nikon, but I don't read that part of the site). Every review I read of a Tamron or Sigma lens reads like a death sentence. "My 2nd copy was better than the 1st, but my 3rd copy was so bad I wished I had the 1st copy back..." and so on. Or "I really wanted to like this lens, but..." I think I have read every Sigma or Tamron review on the site and none of them reccomend any of the lenses. In fact, they they kind of discourage you from buying non-Canon.
That said, I went to the one old-school camera store in my PA city today and tried out the Canon 70-200 2.8 IS II, (and also the non-IS f4 version) and compared them to the Tamron 2.8 equivalent. Have to say, even just tracking the stuff on the store shelves, both the Canons were much faster to focus, and way smoother/quieter.