I did not send my kids to kumon. But I have bought the kumon workbook for my kids to do at home. I find that their maths and english workbook are very repetitive. For example, you do 1+1, 1+2, 1+3 over many pages. They believe that practice makes perfect. But after a while it becomes very boring.
I heard from another mommy that there are lots of homework to do at home. But if your child is suitable for this kind of drilling method, then he could progress very fast. For example, a 3 year old boy can be doing P3 maths.