I would really say it depends on your child. I didn't have a good experience with kumon with my 5year old. I guess it's the age though. We started Kumon in March, mainly because I felt he wasn't being challenged enough at school(reception) he's a really bright kid who really enjoys studying, he always bugged me everyday to give him 'homework' he complained he was bored at school and so we decided to try it out. For the first few weeks, he was so enthusiastic, but after a while, it was becoming quite overwhelming for him having to do it everyday. I tried everything -reward, encourage, bribery, but it didn't help. he began to cry when I asked him to do them and then started saying he 'hated' maths which really scared me, all reports from his teachers even at preschool was that he had excellent mathematical skills, now hearing him saying he hated it.
The final score was when in his sleep one night, he started saying 'mommy I don't want to do kumon anymore'!!! so we eventually stopped after 3months. He's way ahead of his class anyway, I just didn't want it to be counter productive and put him off academics altogether.
Having said that, I think the maths is very good, it makes them think fast and improves their mental arithmetic. You might try it out for your dc who is behind. You and him/her just have to be motivated and hopefully it'll work out for you.