Oct 9 2005. Tested here are 4 popular lenses:
Canon 17-35 f2.8 L USM
Canon 17-85 f4-5.6 IS USM
Sigma 18-50 f2.8 EX DC
Sigma 20 f1.8 EX DG RF
This is a very quick test I put together as I only have the 17-35L for a brief time as a loaner from a friend. See the comments embedded in the image itself. The 17-35L was replaced by the excellent 16-35L but is a very capable performer in its own right. I very much like the build, the silent USM, the fast and quick AF, and the "clean" optics. That said, in my 200-300 frames shot with it, I can't say with any confidence it out-resolves the Sigma 18-50EX on my 20D. But it certainly has the nice L build and refinements.