I don't know whether Kumon is good, for we haven't tried it before. But I strongly recommend Beestar.org.
http://www.beestar.org/index.jsp
It is a website full of free math practice, English programs, social studies and Gifted & Talented programs. My little Emma started school last year, but she really hated the boring math. Then her teacher recommended Beestar to us. She began to like doing the weekly math practice, because there are interesting stories and vivid pictures for kids. She also likes checking the honor roll every week, and she is so happy when her name appears. Registering as parent, I can receive the Email from beestar every week to notice me how Emma was doing in beestar. I am considering registering other programs for her.