Some thoughts:
I did Kumon as a way to get away from the boredom of maths at school. Started it in mid-97 (so this would be Yr 4) and completed the program (Q, which is as far as it goes in Australia atm) by some time which I can't recall.
2001, I think. Given that the stuff I was doing was *ahead* of what I was doing at school, I think it turned out to work fairly well for me. The repetition gets a bit useless past the L/M levels -- the worksheets themselves are good though. As long as you have a supervisor who's willing to give you some leeway in deciding what you need (and most supervisors will), it works fine as an extension program.
From the other side, viewing Kumon as a "catching-up" type of study, it works much better at lower levels than at higher ones. I worked there both during and after my completion of the maths program, and I felt that it really helped some people with getting up to speed on basic calculations (especially the E/F levels, with fractions). As with all things, you did have to put in the work to achieve results though.
Although I never did the English program, I marked a fair bit of it, and I didn't feel that was so useful. It allowed students to keep a check on their basic grammar and spelling, but a far smaller proportion of people seemed to benefit from the program.