Hi all, I own a Canon 40D and am looking for good lens to use as a walkaround lens. I have no plans to upgrade to full-frame anytime soon, and if I ever do, I will still need a walkaround lens for THIS camera. Maybe I am doing it wrong, but I am just buying a lens for the camera I have right now. Money is, naturally, a problem, but I don't want to sacrifice quality too much. This is a walkaround lens, one that will probably take some 50% of my pictures, so I am looking for a good-quality lens that would handle low-light shooting without flash or tripod every once in a while. So here are the finalists: Canon EF-S 17-55mm f/2.8L IS Costs ridiculously lots of money, build quality is ... well, plastic and has every feature imaginable. It's not every day you see something like this. I have concerns about build quality. I had concerns about image quality as well, but so far reviews tend to agree that it is on-par with L-class. Canon EF 17-40mm f/4L Very reasonable price, L-class build, L-class quality, but on my body I will be changing lenses every time I need something slightly on the long side. Clearly this lens was never meant to be used as a walkaround lens on a half-frame camera. Also it's a full stop slower than the 17-55. I have concerns how it performs in low-light. I have read all sorts of reviews, I have gone through all sorts of pretty Excel graphs measuring everything imaginable. Both lenses appear to be pretty good. I would like to account for the "human factor". How many of you have used either of the two lenses and what are your impressions from them? Advice and opinions are welcome. Thanks!