Ah yes, Kumon. A marmite-y kind of experience, in a love-it-or-hate-it way.
Ds has just done a year of Kumon, and I think I'm reasonably balanced about it, so I'll summarise.
It is very good for several things: getting kids to understand about how numbers work at a very fundamental level. So it's good for the child, like my ds, who in Y1 could get enough of the right answers enough of the time to not stand out, but didn't really understand what he was doing and why.
It is also very good for getting children to focus, work by independendently and fast. They set time targets for the work, and the child has to try and complete the worksheet within a given time. They're also very good at incentivising the children -- there are endless stickers, certificates and rewards.
The downside is that it is just pure maths, numerical operations. So you can get to the point that my ds did over the summer where he could work out 20-17=3 like lightning, but was completely banjaxed by, "If I have 20p and I buy a pencil that costs 17p, how much change will I get?" Some children may be able to extrapolate from the pure maths to the language-embedded problems, but I suspect a lot can't. It is, apparently, completely hopeless for dyspraxic children, as they don't learn in that pattern-y way.
It is also, undeniably, deeply boring. I wouldn't choose it for a bright child who was underdemanded at school, for example, as even though children can progress at their own pace, there is a lot of repetition. It's very Japanese, somehow.
Overall, I'm glad we did it for ds, and it has definitely helped him to get a firmer grip on basic mathematical operations. But after a year it was time to move on, and I've bought some commercial workbooks for him to practise with, as his teacher has indicated that he does need extra support at home. It's quite a relief not to have the weekly palaver of hanging around in the cold Kumon centre, and I do have other uses for £45 a month.
hth