I've got the Tamron 17-50 mounted on my Canon 450D and absolutely love it! Was amazed in the difference from the kit lens in sharpness. I haven't tried the Tokina but have tried the Canon 17-55mm IS, and while the IS was great I found the CA shocking, especially at 17mm. Plus the Canon lens is more than twice the price of the Tamron. On a side note, I've got the Sigma 10-20mm and have found it lacks sharpness and really needs to be stopped down to f/8-11 to be acceptable. I was looking at the Tokina 11-16mm as an option instead of the Sigma as it's faster and you only loose 1mm on the wide end. Seems like it would make a nice compliment to the Tamron 17-50 (11-50mm at f/2.8).
Nice review on the Tamron here:
http://www.the-digital-picture.com/R...ns-Review.aspx
Troy