I've been on the "success" diet through Amcal Chemists for the last month and lost 5kgs... but once I stop, the weight just goes back on. It's a tough one. I lost 10kgs on it last year after being on it for 3mths so that was good, about 1kg per week you can lose, initially you will lose 3-4 kgs in one week but that's with "ketosis" which you can achieve by sticking with the diet 100% and NOT having sugar or cheating....and what happens is that your body starts eating from the "fat" stores in your body rather than the "energy" stores in your body. you'll only have 1/3 of the kilojoules you are meant to eat in a day so you tend to feel lethargic at first the first few days as your body overcomes not eating sugar and carbs.... once you get past first week you are fine.
I do believe they are "yo-yo" diets because they aren't long term and once you go off it the weight comes back, even if slowly....

Its good if you need to lose weight for a special night out or something, if you stick with it you will lose 3kgs in one week... i did anyway.... now i am just having 1 shake at b'fast time (its fast too! i can sleep in! hehe) and for lunch have a salad with fish (mackerel tinned) and yoghurt and for dinner steak and veges or meat and veges trying to cut out bread and carbs.... (thats the hard part!) MY DF is still on the diet but i am getting tired of it. it does get "boring" but good luck!!!!