well, i owned a 17-55 f/2.8 for about two years, and recently got myself a 15-85. the little extra reach on both ends was something i seemed to use more than the constant f/2.8.
i usually shoot in AV @ f/5.6 anyway
in the mean time, i have sold the 17-55
image quality is great on both lenses. the only thing that i notice is more vignetting on the 15-85 across the focal length. but it's a simple fix in post processing, if it bothers you.
the 15-85 is still very usable in low light (museums etc... where the objects are not moving), the IS really works very well.
also, 72mm filters are cheaper than 77mm filters