From what I have seen, it is as good as as brand models in terms of Image Quality/Sharpness. I tried it once at some photo fair, its build quality is solid, the zoom ring is somewhat stiff (maybe it is new). Also there is no Full Time Manual Focus, but it is not a big deal. I never tried the Canon's 17-55 and/or the Tamron 17-50 VC yet, so I cant say anything much.
Here are the reviews of this lens :
http://www.lenstip.com/256.1-Lens_revie ... S_HSM.html
From the reviews, this seems to be a really nice bargain, although it the most expensive among thrid party lens.
PS - You get a hood and a pouch/case too!!! Too bad Canon isnt as generous as Sigma / Tamron :p