If you are trying to decide between the 10-22 and the 15-85/17-55, I would recommend picking up one of the latter lenses first. The only time I go lower than 15mm on my 10-22 I would say would be for taking starscape photos where I want to capture a LOT of range in the sky. Get the 10-22 if you plan to spend the night on Machu Pichu, that would be an incredible starscape!!! But 15 or 17 would be perfect I would think for 95% of the shots you would want to take, and I kind of wish I had bought either the 15-85 or 17-55 before I bought my 10-22. I had a 28-135 at the time and thought that with those two lenses I would cover the range nicely. The area between 22 and 28mm on my 7D is probably the area I wish I had the most. I definitely like having the 10-22, but I suspect when my 17-55 arrives it will see less use.

But you have the 18-55 now, so I guess you should have an idea of what 18mm can give you. Good luck!