I've tried the 5:2 for a few weeks, and lots of my friends and workmates do it (also, we're all scientists, so we're pretty skeptical on most fad diets!).
I liked it - I couldn't do a 500-calorie day when I wanted to exercise though (I think that'd be asking for trouble) and as I pole 3 days a week I just couldn't fit it in, otherwise I'd have liked to have kept it up. I didn't want to lose weight, but I wanted to give my blood sugar a chance to properly go down and restart again, which this diet does.
My friends on it all lost a few kilos and then they stabilised - but they were all only a few kg overweight to start with. My boss lost 15 kg on it over 4 months! I think this is good for slow, consistent weight loss, which is what you want really, so it's fat and not muscle or water loss.
On the "diet days" you really have to plan though - you want to eat LOADS of salady type things, so you really make the most of your 500 calories, you can't just have a banana and then nothing for 12 hours. If you plan well you don't feel too hungry, just keep yourself busy. I found that the next day I was a bit hungrier than usual, not much though, and I did feel amazing the day after - like my whole body had been turbo-charged and wasn't the slightest bit sluggish.
Hope that helps! I think it's one of the rare fad diets that are actually good for you, but obviously you have to be sensible - don't pole on a diet day, drink LOTS of water and fill up on things like celery, and eat fairly well the rest of the time too. Give it a shot, a few weeks can't hurt!
