Biggest advantage for me is the wide focal range it provides. Love this lens on my 7D.
I agree. The 15-85mm was my first big lens purchase as a DSLR noob and I really like it. I have read reviews saying the IQ isn't much better than the 18-55mm and that it's overpriced. I took this lens with me on vacation to Washington DC and it was great having the 15mm wide angle and 85mm telephoto in one lens. Coming from a point and shoot, I can't complain about the image quality at all. I wanted the 17-55mm but I don't think I would like to be limited to 55mm with a walk around lens.
I was worried about getting a good copy of the 15-85mm. There are several reports in the forums on DP Review, that the quality of this lens has gone down in the past 6 months.