I use it too. I like to marble when I want to get rid of my nail polish. 4+ drops of polish for one nail? Crazy. Although you can dip more than one finger in the cup at one time, but I like "choosing" my designs.
Also, make sure the nail polish are slow dryers in order for your to swirl around before it "dries" on top of the water. You have to play around with the water temperature to find what helps keep the polish from drying faster.