When I bought my first Canon DSLR several years ago the kit EF-S 18-55mm f/3.5-5.6 II that came with my camera was horrible. The succeeding IS versions that replaced it were much better, but still leave a lot to be desired.
I had already bought an EF-S 55-250mm f/4-5.6 IS lens as well (very good value for the price in my opinion). Maybe I just got a bad copy of the 18-55. Maybe I got lucky with a very good copy of the 55-250. In terms of optics the EF 55-250mm f/4-5.6 IS is pretty much the equal of the more expensive EF 70-300mm f/4-5.6, it just is limited to APS-C cameras and is one half the price of the 70-300. For a more in depth look at the 55-250, see my answer at this question. Here is a comparison of the two lenses.
At that point in time my budget was extremely limited and the solution for me was the Tamron AF 17-50mm F/2.8 XR Di-II. Tamron had an instant rebate going at the time and I picked mine up for about 15% less than the current market price. In terms of optics it is every bit as good as a couple of subsequently purchased lower tier "L" lenses. I got a lot of great images with that lens for several years. If it weren't more or less on permanent loan to my brother-in-law I would still use it occasionally when the need to shoot in that focal length range on a crop body arises. The biggest downside was that it got me addicted to constant aperture f/2.8 zoom lenses! For a more detailed look at the AF 17-50mm f/2.8 Di II see my answer at this question.
You can compare the EF 17-40mm f/4L and the Tamron AF 17-50mm f/2.8 Di II here. At most focal lengths the Tamron is just as sharp at f/2.8 as the 17-40 "L" is at f/4. My own experience with both these lenses has been very similar. When stopped down to f/4, the Tamron 17-50 holds its own with the 24-105 over their common focal lengths. Again, this mirrors my own experience with both these lenses on crop bodies.
DxO Mark comparison. For whatever reason, there is no test data for the 17-40.