Mountain dew leaves the hookah smelling like dust.
. Well, like... dusty metal. It's hard to explain.
I;m sure using Soda in your hookah is not a good idea though. I would think that it would speed the corroding your downstem. Maybe not though.
. But the bubbles make your throat hurt.
I have used flat Coke in Cola flavor, i couldn't tell whether or not it was imparting taste because Cola is pretty pronounced, but i'm sure it didn't take away form the taste.