Very good alternative to Canon Original

Short to the disadvantages that one against the very helmsmen EF-S must take into account 17-55:
- The autofocus is very noisy as it is a rack drive. Thus no FTM is possible and the focus ring rotates with the focus.
- The autofocus is a little slower than USM, but anything but slow because of the focus angle is only 45 °.

That was it but also already to real disadvantages. The image quality is absolutely top, so can be achieved even at maximum aperture in the range of 17-35mm very sharp images. For more focal length, the image is then getting softer, such as what font stands out strongly, especially when photographing high-contrast details. I use this lens but especially for the wide angle that's no problem for me. For larger focal lengths could be 70-200 define the favorable EF f / 4 L USM also. If you close the aperture a bit the sharpness throughout the zoom range top to the edges.
The EF 17-40 L USM, which "only" 200 EUR more expensive than this lens is not an option, since it in Crops (it did at a 400D tested) very soft images produced in all focal areas and although it only when Aperture f / 4 starts.

So I give 4 out of 5 stars, because you definitely have to deduct 1 star for the noisy and somewhat slower AF. What it should arrive but the image quality and the true :-)

Another note: For full format of this lens as EF-S lenses not suitable (even though it has an EF bayonet to identify the red dot), as there were shadowed occur in the lower range of focal lengths and it is to use so until around 24mm.