Decided to give a try to this lens instead of Canon 17-40 or 16-35 as I am not that much into wide angle photography, and did not want to commit too much money for a lens I would not use often. Also, did not feel like lugging another solid and heavy L lens that may or may not get used on a particular occasion. Had this lens for a few month, used it a few times, and have been very satisfied so far.
It is light and compact, but of pretty substantial built, so I just keep it in my bag just in case. Sharpness is quite good, so that I have not had any shots ruined on that account - never tried to use it for very detailed landscape images, but for taking pictures of people and building in close quarters it has worked very very well. Color is very nice, vignetting is not bad at all, have not had any major problems with glare, softness in the corners is not bad, and it is fast at 2.8 between 17 and 20 mm!
Very usefully, Lightroom 3 has a profile for this lens for RAW image processing (not for JPG), which makes it most useful for correcting the distortion present in all wide angle lenses! I never use the hood with this lens as it just takes valuable space in my bag. Quite glad got this one instead of Canon 17-40, as it is faster, lighter, far cheaper, and provides pretty good image quality.