At first, for normal zoom lenses on APS-C cameras, there was only the relatively poor 18-55 kit lens, and the 17-40 as the main high-quality choice.

Then came the 18-55 IS kit lens - a noticeable improvement over the lens it replaced, and the 17-55 f2.8 IS at the high end. The 17-40 fell in between these two in both cost and capability, so it was still a reasonable choice.

Now, the 15-85 offers twice the range, and on average about the same light-gathering ability, while costing less than $100 more than the 17-40. And there is also the more economical 18-135, undercutting the 17-40 on price.

So there are good choices at low, mid, and high price points in EF-S lens form.

The 17-40 only makes sense as a purchase for use on an APS-C sensor camera if two conditions are both met:

1. The user is "just passing through" APS-C on the way to a "full frame" sensor camera, and
2. The user doesn't want to pony up for the 16-35 f2.8, even on a full frame camera.

A big change from a few years ago.