Great lens for the money paid, great colors (comparable to my 70-200 f/4 L), great resolution at 50mm, less at 17mm (maybe only for the wide angle effect, not an expert on this focal length...); on the wide side also the distortion is very low, low CA and low vignetting.
The AF is noisy and miss focus on very dark situation, but is very very fast and accurate; manual focus is good, but the ring has a short angle of rotation, so you need to be careful when focusing. The min. focus distance is very short, so you can get very close to an object; the magnification is not so much, but acceptable for a general lens.
Sometimes the lens don't work well with the Speedlights, I recorded some underexposures with my 430 EX.
I see internally some dust within the lens elements, the gap between the element that extents and the barrel could allow some dust to enter.
The 2.8 aperture is very good, also for portraits (but there are better lens in this regard!).
After 10 months I'm very happy for the money paid. I've already tried the Canon 17-85 IS and disappointed me, especially at the focal length of 17mm (very bad distortion, CA, resolution, etc..). This lens is far better!
Maybe 20 mm more on the long end are better, but surely you loose on quality, so it's better as it is!
just my 2 euro-cent!