Quote Originally Posted by clemmb
I think at all ends of the zoom range they are probably an even trade in IQ,

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.