It worked well for us. In fact even now, about 18 months on, even if Abby is awake, if you go in to get her up before Mr Sun is up, she will angrily send you away!
I think it is important to read the book as a bedtime story along with it though, so they get the idea what it is all about. And re-inforce it. In the early days, when she woke in the middle of the night, we always told her Mr Sun wasn't up so she had to stay in bed.
For those who are early risers and come out of their rooms, maybe one way to approach it isn't that they have to stay in bed until the sun is up, but that they at least have to stay in their rooms, playing quietly with their toys or something.