Originally Posted by clemmb
I respectfully disagree. The 17-55 has better contrast, resolution, vignetting, and more:
17-40 @ 17mm f/4 vs 17-55 f/2.8 IS at 17mm f/4
17-40 @ 28mm f/4 vs 17-55 f/2.8 IS at 28mm f/4
17-40 @ 35mm f/4 vs 17-55 f/2.8 IS at 35mm f/4
Even the lowly 18-55 kit lens is sharper than the 17-40 at some focal lengths/f-numbers:
17-40 @ 35mm f/4.5 vs 18-55 f/3.5-f/5.6 at 35mm f/4.5
It isn't really all that surprising considering that the 17-40 was really made for full frame and shines best when stopped down to f/11 or so.