There is a very good thread in the 5:2 section asking why 5:2 hasn't worked for some people, so I thought I would ask the same question here (also in weight loss chat as presumably not many people will be here if it hasn't worked).
For me, I lost stacks of weight last year low carbing, but as soon as a I had carbs again (not massed, just a few glasses of wine and bit of Easter chocolate) it piled on faster than ever and I am pretty much the heaviest I have ever been (still normal weight, but a stone over where I would ideally be). I'm just not sure how I would have introduced carbs more slowly if circumstances hadn't allowed it - ie holiday/work trip.
So, if it hasn't worked for you, or has worked but you put it (and more?) back on, please tell me about it!