I want to upgrade my kit lens and found a used Sigma AF 18-50mm on a local site for 175$. At the moment I have a Canon 18-55mm and a 50mm 1.8. Or should I wait and get a Canon 17-55mm USM ?
I have the 18-50 DC EX Macro version in Pentax K-mount as my main lens. I like it. Your main issue will probably be the lack of OS (which was added in the 17-50 successor.) I'm not too sure what the difference between normal and macro versions are, other than close focusing, but both should be relatively inexpensive second-hand. Shots with the 18-50, have some: IMGP5944 by silverfish51, on Flickr IMGP2854 by silverfish51, on Flickr IMGP5215 by silverfish51, on Flickr IMGP3824 by silverfish51, on Flickr
I've had one for about a year, it's served me very well for both video and stills. Just on the verge of switching to the 18-35mm 1.8 but just because I could really use the extra speed.
I've got the Tamron 17-50mm and have found it to be excellent, don't need OS as it can be shot handheld in most conditions with wide apertures. http://www.tamron.com/en/photolens/di_II_hi_speed/a16.html
I. Use the 18-50 ex macro lens as my standard lens, used it for years now and love it still. Never needed the OS that came along later . Shots with it can be seen here http://www.flickr.com/photos/whisperwolf/tags/sigma1850mmf28exdcmacrohsm/