I have the VC version of this lens. It's extremely sharp, offers great colors, the VC is pretty remarkable - overall a very good lens.
The only knocks I have against it are related to the autofocus. First, it's loud. I really can't overstate the noise that comes from this lens when autofocusing, especially compared to a USM. It's a mixture of a whir and grind that I haven't gotten used to in the 6+ months I've had it. Second, it isn't as fast as a native lens. But then, that's a problem for all third party lenses. We aren't talking about a huge difference in AF time, but it is noticeable when you switch from the Tamron to a Canon lens. Finally, the AF hunts and sometimes doesn't lock on in low light. It's not a huge issue, but it has happened enough for me to notice it.
If Tamron put their new fast, silent AF motor from the 70-300 in this, it'd be a home run. Otherwise, if you can live with the AF differences, slightly lower corner sharpness, and a slight difference in color (in my opinion), it's a very fine lens at a great price. As for me? I'm currently trying a Canon 17-55. If it's a big enough difference, I'll sell the Tamron. If not, I'll return it knowing I already have a good lens.
