The 17-50mm is a popular choice due to the fact it is cheaper than the canon alternative the 17-55mm. From the reviews I have read in the past the image quality is meant to be reasonably good although very soft in the corners wide open but get sharper when stopped down. The autofocus is pretty quick but will be a lot louder than the usual USM and no FTM. Some people find third party lenses to be not as accurate at focusing as the Canon lenses but in practice it is probably still good. I think the build quality is slightly lower than the 17-55mm, although it does include a lens hood out of the box. The VC is also loud but I would expect it to be relatively good performance wise.
Whilst this lens is clearly not the 17-55mm it is half the price. For around £300 you are getting a constant 2.8 lens with VC. As a result there are drawbacks so you will have to decide if you can live with them. Overall it is a great lens for the money, although I have heard the non VC version is sharper so it might be worth considering if the VC is essential... although it probably is