I will qualify this review by saying that I've only had the Freud SD508 Dado stack for a few weeks.  In that time, I've used it to make dado cuts in Baltic birch plywood, HD 'sand ply' plywood, hard maple, yellow birch, poplar, MDF and MDF with laminate.  In all of these materials, I was impressed with the flatness of the bottom of the dado and the clean edges at the top of the cut.  The outside blades do leave a light scoring mark in the bottom of the cut, but that's about it to my eye.