It's quite clever really, after playing around with the available 'plans' on the website, I can see that on some plans, what you get back (if you complete the plan successfully) is exactly what you paid over the course to do it in the first place. Some plans do actually give you some cash back - but they are banking on the knowledge that a lot of people will not, and cannot stick to a diet. This plan requires you to lose on such a strict basis - and then MAINTAIN for a certain amount of time also. If you drop out for any reason except a certified medical condition, or pregnancy, you will not get any money, will not be refunded any membership fees, and will have to continue to pay monthly fees regardless of you not being on the plan anymore.
So, while it may be a good motivator on the surface, and may work for some - I'd say read the fine print first.
I agree with Minerva, although its a shame that last January I didn't have a crystal ball to see into the future - if I had I would have joined and would have earned £655 less fees of £330 = profit of £325!!