This is a very nice review of the Tamron 28-75 f/2.8, compared with the Sigma 24-70 f/2.8. They are both interesting alternatives to the Canon-Nikon mid-range pro lenses. A recommended reading from the Todd Owyoung blog:
All the Todd’s reviews are very interesting “hands-on” tests, direct, deatiled and easy too read.
For the italian readers, an evergreen from the Juza forum