The 15-85 is an excellent lens for what it is, provided you recognize and accept the compromises.
With a ~5:1 zoom range, there are some. It's a bit soft off-center wide, but there's little reason to pixel peep wide. (Why would you crop a wide shot instead of zooming in?) CA is also pretty severe on the wide end but that's easily corrected in post.
The edge-to-edge sharpness is much better at the narrow end. Center sharpness is good in general.
There are VERY FEW lenses with a 5:1 zoom range that perform anywhere near as well as this one does. It is also light, small, and has an excellent IS implementation, which just adds to the attractiveness, and it is also reasonably priced.
I contemplate selling mine on occasion and keep coming back to "do I really want to have to carry two lenses and spend the space and mass to do what I can do with one now, when I want a nice, light and small walk-around solution for a day of shooting?"
So far I'm keeping it.