My dt's do kumon maths and they each benefit in different ways. They are at a state school, they do a lot of literacy work but the school seems to suffer from some kind of anxiety when it comes to teaching maths and arithmetic in particular. They are a good school and offer booster tuition in KS2 for those that need it. So my experience would be don't do kumon if your dc is struggling already before you have checked with the school.
But I have been really pleased with what it has done for my two. They are very confident about numbers, they understand the importance of regular practice, it helps ds in particular to focus on a task (and actually it seems to help that the task isn't v interesting, in a way) and it gives me some transparency on what they can do.
I asked DD the other day what they had done in numeracy and she said "positional vocabulary". So forgive me, but I would much prefer that my dc's spend time practicing addition and subtraction, than demonstrating that they understand the difference between "above" and "below" and that they can parrot DCSF jargon.
Practice makes perfect with reading and writing, so why not with numerical operations?
I am not doing it for teaching concepts, but for regular and structured practice. It might not take long to replicate, but I place a higher value on my time than what I pay kumon for the worksheets. Plus like most children, they would rather impress someone else than me.
We will stop when they are older, I expect, because I'm not sure you get the same benefit from repeated practice at more difficult maths and I imagine you would actively want to have to do it.
So, in short and in my opinion, kumon has its place - it depends what you want it for.