I've done lighterlife which is similar to Cambridge and lost 3 stone in 3 months. It actually wasn't difficult after the first week, it's a no brainer not having to think about meals but, yes there is always a but, the problem comes when food is reintroduced. The weight loss is so quick but there is no real re-education about how to eat. You get a lot of advice but not the practice. Wham straight back to eating in the way that caused the weight in the first place!
Also. I know a couple of people that have had serious health problems afterwards, one ended up in hospital with arythmia and was told by the doctor that it was definately caused by the diet, he said they see it a lot with very low cal diets, 500kcals and less.
Slimmimg world seems to be a great way to go, but I've only just started it so not proven to me yet.