I also had a bad experience with Flat Worm eXit but not nearly that bad. My White Cheek Tang and Foxface had marks on them like burns and stayed hidden for a week. But, they fully recovered and the flatworms are back just as bad as before. While visiting Hide Tide Aquariums in Tallahassee this last week, they said that they have no problems using it but they perform a 70% water change immediately after the flatworms are killed. I might try again if I can find enough containers to make that much water.
By the way, if you ever get to Tallahassee check out High Tide Aquariums. It is a beautiful store with tons of corals. Very clean and neat.