Total baloney. They put stuff like psyllium husks in those things to make you think there's evil goo clinging to your bowels. The psyllium becomes mucus; your body is not hanging on to some disgusting goo.
I had a colonoscopy almost two years ago. If you want to clean out your colon for some reason, just buy two boxes of Fleet Phospho-soda. Use one whole box, then in 12 hours use the other. Don't eat anything in between. There. Colon cleaned.