I have used this lens for about 6 months and finally feel like I can write a helpful review.
I did a lens test against my Tamron 90mm f2.8 macro (supposed to be one of the finest lenses available for Canon EOS). I shot on a tripod, timer release in good light at a flat piece of paper. I tested several focal lengths on the Tokina, framing each one to the same view as the Tamron 90mm. I found that by f7.1 the Tokina is basically as sharp at 100% crop as the much praised Tamron, at all focal lengths, even 24mm and 200mm. (Note, I didn't test between f5.6 and f7.1, so maybe The Tokina even becomes "Tamron 90" sharp before f7.1.)
BTW, the Tamron is supposed to be as good/better as most any Canon "L" zoom lenses (based on Photozone and Photodo ratings), and if the Tokina is as good as the Tamron, well...
Perhaps I have a great copy of the Tokina, who knows. I believe my Tamron is a "good copy" because of the pictures I'm getting out of it both in tests and real world pics.
I would highly recommend this lens, with two caveats.
1) This is NOT an indoor/low light lens (unless you want to use a flash or tripod). I live in California, so sunlight is not usually an issue; I love using this lens outdoors.
2) I haven't tested the Tamron or Sigma 18-200mm lenses, so I guess if those have the same optical quality as the Tokina at f7.1, then maybe they would be a better choice due to getting more on the wide end. Of course, both the Tamron & Sigma are $130 more than the Tokina, so that's something to consider as well.
I would challenge anyone to suggest a better lens considering the price, focal range, and sharpness (when stopped down).
One last thing. Let's compare the Tokina to the MUCH beloved Canon 70-200mm F4 L, which would cost you $600. So ask yourself, is Canon's sharp at f4 versus Tokina's sharp at f7.1 worth losing the 24-70 range and paying $300 extra dollars? And the weight of the Tokina (which many complain about) is exactly the same as the weight of the Canon 70-200mm f4L.