I shoot with a 40d and do most of my photo editing with google's picasa. One of the first steps in my editing work flow is to press a single button called
'One-click fix for lighting'. This utility evaluates the lighting and adds to the Highlights and Shadows as needed.
Around 75% of the 42k shots I've taken with this camera are with the 18-55 IS. In the vast majority of cases the editor added significantly to the highlights and a bit to the shadows. But now that I'm shooting with the 15-85 I find that the editor rarely adds any to the highlighting or shadows.
I'm thinking two things are involved here. One could be that the communications between this lens and the camera is better than with the 18-55. But more likely that the improvement in glass results in better images. I guess better contrast is the term needed here.

So upgrading from the 18-55IS to 15-85 gives:
Better range
Very slightly better sharpness
Better control of CA
A little bit more distortion at wide angle
Better contrast

Andy
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http://lh3.ggpht.com/_fOoZcVpIqxk/SZ6x9qiyXKI/AAAAAAAAMDY/vocEInekhwU/s288/LightningWide1.JPG