The children in my class absolutely love it and beg to use it during their computer time in school. I have used it to set homework but don't any more as many children do not have internet access. I have noticed, from checking on the teacher's page, that many children use it almost daily at home. They often come in to school and ask me if I could set a new/higher challenge for them. I love this enthusiasm.
Anyway, I'd definitely recommend this to any teacher. It doesn't take long to register, it's totally free and the children are mad about it.