The old Grapefruit Diet has been floating around for a long time. Its popularity for so many years can be attributed to pseudo-science and good marketing copy. This is one of those fad diets that you can use to drop some quick pounds, but, like all fad diets, you need to recognize that it's deficient nutritionally and that you'll gain back what you lose as soon as you get off the diet IF you return to your old eating habits.
It's pretty much agreed by the Grapefruit Diet gurus that most people who try the program for a week or two will probably lose somewhere between 12 and 20 pounds.