My DD did Kumon from the age of 5 till 11, I instigated it because she was good at maths and enjoyed it. Although it seems to start slowly, the more understanding and ability the child has, the more quickly they rise through the levels (not always possible in school, which aren't always able or willing to accelerate an able child to the point where they are being stretched as much as an average child.)
DD is now 18, has a place at Oxford for October, and has always felt entirely confident with Maths, something I feel is due in part to Kumon. My summary would be:
It's a long-term commitment - you may feel it starts too slowly and too easily
Children are not that keen to do it - even my maths-loving DD was not enthusiastic when her worksheets came out every day - even though the work should take no longer than 20 mins or so - and NOW she can see it was worthwhile
It doesn't cover every aspect of maths - but is brilliant at ensuring facility with both simple and complex calculation (and thus giving great confidence to tackle problem-solving)
It does very successfully (after the early months) give any child the chance to work at a level she is capable of, despite her age - even if this means quadratic equations at 9. Evaluation and assessment is continuous and always with an eye to moving the child on when ready.
It is VERY expensive - but I'd say it does offer something you will get nowhere else, it's likely you just won't have the discipline or inclination to achieve what Kumon can by working on your own with your child.
So overall I'd say if you can afford it and are ready and willing to keep your child doing it daily despite some reluctance, and if you won't get impatient and expect short-term results, it's well worth it. (I wouldn't say the same about Kumon English, which we also tried, and I felt just doesn't lend itself as a subject to the Kumon system the way maths does.)