well from someone who has tried an 800 calorie diet i can say that yes it does work but it is extremely difficult. i don't actually know anyone who was able to successfully keep to it (unless they had a gastric band), I managed to last for a month. you notice a difference within 4 days, my clothes had become so loose. but a warning - the worst part is not the weakness or light headedness but the PAIN. at some point (for me it was after a week) you will feel the full effect of hunger pains. and i'm not talking about normal hunger pains here, i mean you will have extreme stomach pain for a while, it will feel like your stomach has turned to stone.
and thats usually the part that people find the worse to handle and thats usally what makes them give up. if you can overcome that part and still stick to it then yes, of course it will work. its dangerous and can kill you (thats why people tell you that it doesn't work) but it does work, of course it does. how the hell else did children in africa become so skinny?
also be warned that if you give up and go back to your normal eating habits you may actually end up eating more than before therefore becoming bigger than you were to begin with. most ideal weight loss method is to eat less than you are eating now (say cut out 500 calories from your current calorie intake) and exercise more than usual and daily. thats worked successfully for most people.