I went through similar decisions when buying recently. I went for the 15-85 and I love the pictures I am getting, but at times indoors I wish I could have the 2.8 available. I use the whole range of my 15-85 so it is a tough call between the two. Like Kayaker, I didn't realize how much I was going to spend, but now I see myself wanting the 17-55mm f/2.8 for indoor and something like the 24-105mm f/4L for outdoor. I am sure I'll be asking for advice soon here.

If you are looking to just replace what you get from your 18-55mm f/3.5-5.6, the 17-55mm f/2.8 should be good, especially if that is what you use primarily. If you are looking for something to cover both ranges you have to decide between the range and the low-light ability. The suggestions I see is to use Lightroom to look at what most of your pictures are at to see what range you will be happy with.

I keep thinking about a prime as well, but on a crop, I feel the 50 is too long and I am not sure I could do without the zoom trying to catch 3 kids in the house. Good luck, reading the forums here have been a big help for me (even though it seems to be increasing my costs).

Eric