IQ-wise, i don't think it's a clear winner. some say is better than the tamron, others say IQ is about the same. i chose the 17-55 over the tamron for the image stabilization (e.g. shooting wide at 1/6 sec or tele at 1/15 sec) and the USM. Note that in P-mode or A-mode, with auto-ISO, the camera's AI may not necessarily allow you to access the slower shutter speed (the AI prefers to increase the ISO instead), without manual intervention. By manual intervention, I mean setting the max ISO limit, or switch to Tv-mode or M-mode (I use 550D, not sure how similar this is on other models).
