It will give you soft, runny stools, but so far everyone says they still have complete control. After a week or so, the stools start firming up. Just do the complete bottle, and after that switch to a fiber-based cleanser and perhaps enemas.
Remember, since you have IBS, your goal is not only to cleanse the colon but to also soothe it and reshape it. That means at least 3 months of very regular, safe diet of mostly cooked foods and no iced drinks, and regular, small packets of fiber passing thru the colon so it can untangle itself, as it were. :)