I considered the 15-85, but for that amount of coin, I didn't like the variable aperture.
But, that is just for my type of shooting, its nice glass, for sure, just not at that price for me.
My 15-85 is a constant aperture. I shoot Av at f/5.6 and just leave it there.
I hear good things about the 15-85mm, but you're creeping into that price range of really excellent lenses. I'm skeptical that it's good enough to justify the cost. Maybe when the used copies come down in price in a few years I'll slap one on my (by that point) outdated 7D.
The 15-85 is an excellent lens. Just as good as my other lenses. It fits perfectly between my 11-16 and 70-200 2.8 IS.
There is no other single lens, at any price, that gives you the same IQ and usability .