I haven't done the C9 Diet (which is made by a company called Forever Living?) but one of my good friends endured it.
From my understanding it works on very similar principles to other very low calorie crash diets. I recall my friend saying that she was consuming around 1100-1300 calories a day whilst doing the C9. She took delivery of a high-tech looking box which had various potions and tonics but I think the main gist is to substitute meals with protein-type drinks and then have an evening supper or dinner.
My friend lost weight, and I think it was around the 5lb mark. Having said all of this, after a few days she was complaining of hunger and becoming very irritable with her husband and three children. She and her husband dined with us here on one of the evenings during her C9 Plan, and I recall that she had the most appalling bad breath, which I'd never noticed - before or since. I also recall her ravenous hunger and then she felt guilty after she'd eaten a very small amount of pudding. I felt that she was torturing herself.
In the last few weeks she's actually not only regained the weight she lost during the plan, but also gained one or two pounds, which isn't exactly thrilling her. Rapidly putting weight on again after a crash diet is common, and is usually triggered by the famine-feast response. In summary, yes she lost weight (for a few weeks) at the cost of alienating her family and stressing her friends because she was so fractious. She had atrocious hallititosis and constantly fantasised about eating fried bacon. Oh, I forgot - it also caused explosive diarrhoea for her, which meant she had to dose up on Loperamide for a week and not stray too far from the loo. And now she's over £100 poorer and 2lbs heavier.
Everyone is different I guess, and what didn't work for her might work for you!