I checked with Kumon and e.nopi. It seems to me that they are more like a math tutoring service than a special way of teaching math. The key to both is self-practice, practice and practice. The practice questions that they provided could easily be found in any math book. The role of the teachers is to help the kids with the problems that they do not know how to solve. So it is practically the same as hiring a regular math teacher.
I think the value of this Kumom and E.nopi is providing a convenient math tutoring service for those parents who do not have time to follow up with their kids' homework.
Since I will follow up with my son's homework after work, I would rather buy a math book for him to do at home. It only costs a little bit more than a hundred dollars.