The short answer is yes, it's a significant step up in image quality. You may need another lens to cover the 17-24 range, though, as 24mm on a 40D body is 'normal' focal length, not wide angle. You might also consider the EF-S 17-55mm f/2.8 IS USM, which has L lens image quality and preserves the wide end of the focal range (but then you'd obviously lose the >55mm part). If you have another lens to cover that (e.g. a 70-200 L zoom), you'd be set.