I decided to get the 15-85 IS, and it arrived yesterday. I looked at the 17-55 IS, but didn't like the build quality, and don't really need f/2.8. I also looked at getting another 17-40, but I wanted a lens with IS which I have found to be very useful. I was thinking about getting another 24-105 IS and 10-22, but I'm trying to limit my lens collection this time around (I switched over to Nikon for a while, so now I'm rebuilding my Canon lens collection). My copy doesn't show the flare problem that the one I tried at the camera store did, so they must have had a defective copy. This lens seems to control lens flare pretty well from the quick test shots I done with the lens, and it's nice and sharp! There is a bit of CA though, but it's easy to fix.
I've owned allot of different lenses over the years including L series lenses, and Nikon "Gold Ring" lenses, and I am pretty impressed with the build quality of the 15-85 IS, nice and solid, and feels great paired up with my 7D. I had the 18-135 IS lens too (I bought the 7D kit) and that lens wasn't to bad either, but I wanted a USM lens.
While this lens isn't a L series lens, the picture quality is as good as some of the L lenses I've owned, The only thing that worries me a bit is the amount of Vignetting! But my 24-105 IS showed vignetting on my 5D MkII, and I usually shoot stopped down (I shoot landscapes and stuff like that) so it shouldn't be to much of a issue for my what I will be using the lens for. I'm still waiting for the lens hood to arrive.