According to reviews and several customers, the Tamron 17-55mm is a very good lens (some of those, who have it are claiming that it rivals the Canon 17-55mm), and much-much cheaper, but without IS. If IS is useful in your photography, then try to go for it; however, it may not be of importance for you (shooting moving objects, shooting much from tripod).
There are several areas I am not perfectly happy with the Canon EF-S 17-55mm IS:
1. strong CA (not alway correctable in ACR),
2. strong vignetting (this is a curse with panoramas, though it could be corrected in ACR if I made a one-time huge effort to evaluate the required adjustments at different apertures and focal lengths),
3. it is the sharpest @ f/5.6. This is good sometimes, but in landscape, when I shoot from tripod and want to increase the DoF, the loss of sharpness is noticable already at f/11 (this is not diffraction yet!), the smallest aperture I dear to use is f/13. From there diffraction rules anyway.
Still, my best lens before, the 50mm f/1.4 has been unused since I got the 17-55mm.