Hi there,
definitely worth it.
Once you see it, you think you could probably make up your own exercises for your child, that's what I even tried (and I also used those practice books you can get at WH Smith, but they progress far too quickly for my children). But at the end of the day, I let it slip and did not do it every day, so it's not the same.
We started our children particularly as they are in a state school where they get hardly any homework. You can probably get away without doing it if your children are in a good (!) private school and get decent homework every day (and often at private school as I've heard from friends).
Having said that, at our Kumon centre there are still quite a lot of privately educated pupils so their parents probably have a reason why they send them there.
I remember my mum doing some extra work nearly every day with me when I was in primary school, on top of the regular homework (and we did get homework every day even though it was a state school, but it was in Germany) and it helped enormously.
My ds 1 has SEN and without Kumon he'd be even more behind in school. He's been doing both Kumon Maths for more than 1 1/2 years now and English for a year.
Ds 2 just started Kumon a couple of months ago and already says it helps him with his maths at school.
At school they focus more on the creative side of things, which is fine, but I think it ties in nicely with the more repetitive nature of Kumon.
However, it may not suit a very clever child. They say even those children benefit from Kumon, they just move through the levels a lot more quickly and get harder work, but I can imagine they might get bored after a while.
I don't know if my children will go to Kumon forever, I may switch them to a tutor later on. The downside of Kumon is that it is very pricey and also that you are expected to do the corrections yourself every single day (I admit I was quite shocked by this fact considering the price but have quickly become used to it.)