Ive never tried the Cambridge diet but do know someone who has and it was very suucesful for them but she was dedicated. As with the Lighter Life, when you have lost the weight you need to you must follow the 'getting back onto solid food' plan because if you just start eating as normal again you will gain lots of weight back. However, if you start the maintenance plan when you finish you will introduce solid food slowly allowing your body to reclimatise itself. Whilst you may gain a couple of pounds, if you are strict until you are finished you should be ok. It's those who do not follow the slow introduction of food who regain all the weight they have lost
As for Xenical/Alli (both exatly the same pill - Orlistat) - this has been available OTC for quite a while now and is not new at all, it is just being marketed as a new pill (aren't they all!). The lower strength would be the better one to start with as the side effects are awaful - you will have anal leakage or thick, yellow oil that stinks to high heaven. To be 'entitled' to use this pill, the pharmacist will weigh you and meaure you and also take your BMI which must be at least 28.
Personally, I would see my GP first - on TV last night they were stating this pill cannot be used if you are diabetic, however, my best friend used this as her GP studied all the research and prescribed it for her as a perfect candidate - she is diabetic.
Using these pills correctly, which means following a low fat diet and doing regular exercise, will enable you to lose and extra 1lb per week oin top of the 2lb that you would lose following a low fat diet. I actually feel that it's not worth 50 quid per month to lose an extra 4lb! However, some people do, that would be your decision