I have no first hand experience. But I have talked to some people who have done it and they either loved it or hated it.. The people who loved it were the people who could eat less carbs and handle the slin well, the people who hated it were the people who have to eat alot of carbs when taking slin. They said it made them go smooth, probably simply because they had to overeat on there carb load.
If I were ever going to use slin during my carbload I would definately do a couple practice runs prior to the actual competition.