I love the 17-55mm f/2.8 IS EF-S lens. It's my favorite general purpose lens for my 40D. After that comes the 24-105mm f/4 L IS that gives more reach but gives up at the wide end on a APS-C camera. My other most used lenses are the 10-22mm EF-S wide angle, the 70-200mm f/2.8 L IS and the 100-400mm L IS zoom. Between them, they cover 90% of my photographs. But the most used is the 17-55mm. I don't think you would regret buying it at all, the image quality is wonderful and it is very versatile. Only in terms of ruggedness does it compare less-well to the L lenses, but it is still well-built and perfectly strong enough for my uses (mostly landscape and architectural photography).