I'm not going to say anything about photozone, because it will only be derogatory.
If you look
http://www.the-digital-picture.com/R...0&LensComp=398
You can compare different lenses on different cameras (Not always the one you want mind you).
The corners, that Flake is so concerned about, are actually not as bad as Photozone make out. Comparing it to the 17-55 and the 17-40 is the better bet.
And in the real world, where it actually matters, the 17-40 is fine. And on a crop will be slightly better (You can compare the two on the above site with a 50D)
Finally, I do agree with Flake on one point at least. What's in a letter? "L" is stands for Luxury, nothing more.