My son is not a math kid, and so we did not expect him to compete in math competitions. At the same time, we also wanted to train him on maths, at least quicker than the pace in his school. He is not super good at maths compared to math kids, but was definately way above average than his classmates. So we chose Kumon.
The good thing is that Kumon training helped him to be leading in maths in his class. However, we noticed one major limitation and would like to remind others using Kumon. This limitation can be avoided, i believe.
Based on the Kumon style, at least in our area, students work on questions. If they cannot solve it this time, that is fine, they can work on it next week. If still not good, work on it again in the next week. It has its advantage. However, a big problem I noticed that happened to my son is that he is used to having questions that he cannot understand, and did not work on it. In other words, he got the misunderstanding that the problem will disappear with time passing by. So even in school courses, for some difficult questions, he just ignored those questions. We started Kumon when he was in 3rd year in elementary school and noticed the problem in middle school. He already formed the bad habbit: ignoring the tough questions. It took us some time and efforts to correct him.
Maybe it is just my son's own problem; but I would like to remind parents here. If we knew it, we beleive we can still let him to take kumon and at the same time to pay attenetion to the problem.
My 2 cents.