I just don't get this diet. It doesn't take account of exercise and I bet it's not sustainable for long periods, as 500 cals for two days is too punitive, imo, for most.
I'd much prefer Myfitnesspal, which allows some flexibility and stops mindless eating, which is where most people go wrong when trying to lose weight. I'm sure there are health benefits to fasting, though, but personally I can't stick to 500 cals, which is basically one small meal.
I think it's far better to eat sensibly all the time. As long as you don't snack on fatty stuff and get lots of exercise you should be fine.
And a diet and fitness regime is for life, not just for the duration of 5:2.
Hats off to those who can do it, though.