I tried it based on the Michael Mosley documentary, so using the 5/2 method.
It's very, very effective. But you have to do it properly, which means restricting yourself to 600 calories for two days of the week. This is fuck all food.
I actually found it not too bad while at work; I'd just concentrate and push through lunch and get quite a bit more done than usual. But the evenings drag on and you find yourself just wanting to go to bed so you stop fantasising about food. Surprisingly energy levels aren't bad; after a day of basically no food and a tiny dinner I'd go and play tennis for a couple of hours without struggling.
The morning after a fasting day (and particularly so if you do the two days consecutively, which is even harder but gets it out of the way) you feel a lightness which is really quite amazing, and so unfamiliar at first. Like your body has been clogged with a whole load of excess uneccesary crap which has suddenly evaporated. And god damn does food taste good. After only a day or two of fasting, you really, really appreciate the taste of that food.
You need to approach it like training: the first couple of weeks really, really hurt. You don't see proper results for a month or two, but the weight drops off you and for the bulk of the week you can eat whatever you want.
I have stopped doing it though. Since the first time I did it we had a second kid, and with two kids and work etc I just haven't been able to commit to it properly again. Evenings are too packed full of family stuff, and sometimes you just need a big bowl of pasta and a glass of red for sanity. And to make it really work, you have to stick to it, no cheating.
I think it's worth a go, even just for the sake of testing your resolve. It does make you think about food differently, and you will lose weight. But give it 4-6 weeks of hard graft.