So did the reviewers then. Either not a great lens for FF, or a lot of bad copies floating around.
Photozone 17-40L review on APS-C: The Canon EF 17-40mm f/4 USM L showed a very good to excellent performance in the lab as well as during the field tests. The lens exhibited very little vignetting and excellent resolution figures.
Photozone 17-40L review on FF: The lens is able to deliver a very high resolution for most of the image field but the corner performance is poor thus spoiling the game here...However, these rather critical comments apply to the 17mm setting only and from 20mm onwards it's actually a good to very good lens without any major weakness.
SLRGear.com on APS-C: The Canon 17-40mm f/4 L is a high quality lens with a full-frame image circle that showed really exceptional performance on the EOS-20D we used to test it with.
These are not "just about acceptable" results. They're good to very good on FF, and very good to excellent on APS-C. And while the 18-55 IS kit is surprisingly good and should not produce significantly worse results then the 17-40L, it shouldn't be significantly better either.
I've seen very good large prints from FF + 17-40L. Granted, it was stopped down. And granted, I believe crop + Tokina 11-16 would actually yield more detail and sharpness at the edges and corners. (Talking about 21 MP vs. 18 MP sensors.) But it's not night and day, and the 17-40L is neither a bad lens nor a lens incapable of 24" prints.