It does work as a healthy and motivating jumpstart cleansing diet. You can lose up to 10 pounds within 1 week although it is partly water. But this effect motivates many people to switch then to a healthier lifestyle - exercises included. Many people report that after the cabbage soup diet week they become more conscious eater and increased vegetables & fruits in their daily nutrition what in return lead to even more weight loss in the long run.
If you want to try it - make sure to use the healthier versions with added protein to avoid losing your muscle mass instead of your fat depots.