I know a number of people who are doing this diet (on the internet..not personally)
I don't know enough about sert1 to comment on those benefits, but apart from that I see a number of pros and cons.
Pros
Easy for someone who has done a VLCD. Doing a day of 500 cals is cinch
It's easy to work into your life. You can plan your UD (up days) to suit your social life. Eat what you want when you go out without a thought.
Nobody outside your family need to know you're doing it, unless they see you daily at mealtimes.
Good for those 'all or nothing' people if they are happy to stay in the 'all or nothing' way of eating.
Most people seem to lose weight on it and consider it a really easy diet to do.
Cons
Certainly not suitable IMO for someone who binges. Though people report that at the beginning of the diet they tended to binge on their UDs most seem to go back to eating normally. I think a real binger would continue to binge throughout the UD's of the diet.
Though they say this is a natural way to eat, I don't think its really natural in the western world in the 21 century.
You could say that our bodies are the same, regardless of what century we are living in, but our way of life means we need fuel on a daily basis. Our ancestors probably had many more days of rest, punctuated by lots of exercise chasing those buffaloes for dinner
It would be tempting to eat unheathily on your UDs and leave yourself short of nutrients. You should still need to eat healthily on your UD, though from what I gather, they say you eat what you want.
Having said that, I haven't read the book and I guess many people doing this diet haven't either. The book may recommend eating certain food groups on the UD's.
They do suggest you have a protein drink on your DD's, but I think it's mainly to do with have something that will leave you feeling less hungry, that is pre calorie counted to reduce errors with estimating
The emphasis is on making that DD under 500 cals and you have to be strict with that. Your UDs can be what they like.
I think the diet needs doing with care. It's not going to help you with any eating disorders:sigh:
Having said that, it would suit some people who could do it sensibly. Especially in the short term. Perhaps after a holiday or Christmas, if you are keen to lose those pounds earlier than April