Keep seeing posts of how the 17-40 is wasted on a crop camera. Don't understand why if it suits the purpose it was bought for. I use one in mine regularly to bridge the gap between two lenses and get some cracking results thanks.
I think this has been covered in other posts now. Of course it will work fine on a crop camera, but there are major benefits to be had from a lens that has been specifically designed to cover crop.
The best comparison is the EF-S 17-55 2.8 which, for very roughly similar money, is at least as sharp some say slighly better, has much more range, runs to a full stop brighter (a feature that usually doubles the cost, eg 16-35L 2.8) and has IS as well. Frankly, if it didn't have that magic red ring on it, I don't think the 17-40L would get half the attention that it does from crop format users - the L bit has more do to with build quality that anything else, lots of other lenses are optically just as good.
On a full frame camera, it is a fantastic lens, and as it happens I'm selling all my crop gear right now and getting a 17-40. However, for those folks who maybe buy a 17-40L with a view to going full frame in the future, when you do it changes from a modest kind of walkabout lens to a spectacular super-wide like the EF-S 10-22 and you then need to buy a 24-105L or 24-70L to replace the walkabout range.
For those people shooting on a crop body and buying lenses with an eye to full frame as well, I think they are not making the most of what the smaller format has to offer and it is better to buy EF-S lenses and then sell them on when the time comes (as I am doing) rather than have a compromised outfit. But that's just a point of view.
BTW, the optical benefits of the crop format quickly diminish over about 50-60mm, which is why Canon doesn't make longer EF-S lenses.
Edit: if you buy crop format lenses with half an idea that you might sell them in the future, buy Canon. Stuff like the 17-55 2.8 and 10-22 will hold their value much better than the equivalent Sigma or Tamron.