I'm going to speak in favour of Kumon, it does work in some circumstances.
Before I get shouted at, I'm also a secondary trained Maths teacher and have also taught specialist "catch-up" maths in KS2. Both my children studied Kumon for two very different reasons. My elder child from yr2 on the advice of an Ed.Psysch as he needed to see the work (rather than the ridiculous amount of verbal only in his school) and repetition. My youngest started the week after her 4th birthday because she ASKED to start and really enjoyed it as Nursery maths was way below her ability and she was bored. I did stop once they had covered all the basic arithmetic up to fractions and they both now thank me for making them do it.
I also now see working as a teacher that many pupils do have excellent problem solving skills and can tell you verbally how to solve a problem but sadly cannot reach the correct answer or are frustratingly (for themselves) slow due to poor basic arithmetic, including times tables. I have met many children who are really being held back in maths once they reach yr 6/7 by their lack of basics. Remember they still need to do a lot of arithmetic without a calculator in KS3 and a calculator is only as good as the person pressing the keys, you still need to be able to estimate the answer.
Some Primary Schools are good at getting children to do fast accurate arithmetic but sadly many are not and often it is the children in the lower ability ranges that suffer the most, repetition can really help in some cases. At the school my children were at the numeracy teaching was terrible, shortly after stopping Kumon we moved county and had they not done Kumon even my bright child would of struggled to keep up at her new school.
I also had a friend whose child had SEN and the Kumon centre did adapt the work for them, the SEN child made the most amazing strides. The centre we used never did hard sales to peoples houses, they preferred you to come to the centre and talk to other parents whose children were using them.
My children also did baking, swimming, reading and a lot of time using their own imagination playing on their own.