I've not tried the diet, for obvious reasons. It is just another one of those gimmicky, short-term band-aid solutions that, without a commitment to proper nutrition and exercise, can help someone initially cut weight but put it right back on if they revert back to the same old habits.
Here is some information I found about the cabbage soup diet:
The Cabbage Soup Diet does offer fast weight loss, but it is completely lacking in nutrition, flavor and variety. You'll consume mostly soups of cabbage, onion, celery, carrots, tomatoes and mushrooms. Nutritionists argue that the pounds shed are more likely attributed to the dramatic and dangerous loss in calories and not the proposed fat-burning power found in the cabbage. This diet deprives your body of essential nutrients and has been correlated to difficulty concentrating, feeling light-headed and weak.