I don't use it but DBF does. He was nearly 170kg as is now a littel over 140kg. So he has lost about 25kg in about 12 weeks I think (along with joining the gym).
It's been great as an educational tool for him. He can actually see what's in each thing he eats and how much he can eat.
He raves about it and is constantly trying to recruit others to it.
It did take him a while to figure out how to edit his page but now does it with his eyes closed.