My DS1 (7) has been hugely into them though we haven't read any for a while. When he was 5 he loved them and he was saying the other day that we should get the ones we haven't got.
I was put off them though because they're too sodding short. When I was reading them at bedtime, we'd get through one in a few nights. Still, he can read them to himself now, and I guess that will take a lot longer...
I love the world map that he jumps into. I want one of them.