Toujour dans l'unique but de faire avancer le smilblik, voici le test de Bob Atkins. Bonne lecture

Pour les plus feignant :
The Tamron SP AF17-50mm F/2.8 XR Di-II LD Aspherical [IF] is a good lens. It's pretty sharp at all focal lengths, even wide open, but contrast does go up a notch when stopped down from f2.8 to f4. Chromatic aberration and distortion are generally well controlled, though there is noticeable barrel distortion at the wide end of the zoom range. Chromatic aberration is worst at 17mm, though you can see from the test image above that even at 17mm in the corners of the frame it's pretty good. At longer focal lengths you'd need to look quite hard to see any problems.
..........
As usual the decision about which lens to buy is a matter of compromise. You can have good, fast, optics, you can have IS, you can have a lens for about $500, but you can't have all of them in a single lens! If you're after high optical quality, a fast lens which can blur backgrounds and stop action but you don't want to spend more than about $500, the Tamron SP AF17-50mm F/2.8 XR Di-II LD Aspherical [IF] is a lens you could be very happy with.

EDIT : Pour info, il n'a pas encore testé le 17-55 IS...