Since the thread's been resurrected, I'll say this about the 15-85. It's a sharp lens but it's slow. I recently picked up the Sigma 17-50 f2.8 OS. Less range, but guess what? Sharp as the 15-85 is, my sample of the Sigma is sharper. And it goes to f2.8. It's still very sharp in the center and acceptable in the corners at that aperture. It's about the same size and weight as the Canon and a bit less money too. If you don't need the range, I'd go with it. It'll save you about $400 over the much larger and heavier Canon 17-55 IS too and from what I've seen on the web, it's just as sharp.
I'm a guy who in the past has always gone with the Canon offering. But this Sigma opened my eyes. The image quality is very similar to my 70-200 f4 IS. The 15-85 isn't quite there.
The 24-105mm L is excellent on a crop, but I still took my 17-55mm along because 24mm was not wide enough in tight spaces. I now have a 15-85mm EF-S, and it is a better walk around lens for me, however, if you do not use focal lengths less than 24mm, the 24-105 won't disappoint you.
