There was a recent systematic review into Methylphenidate use in ADHD. It’s reviewed in this NHS choices article.
The authors conclude “we cannot say for sure whether taking methylphenidate will improve the lives of children and adolescents with ADHD”
It’s important to be clear what this means: because the evidence is not of a very good quality, when considering starting this medication, it’s important not to be too optimistic about response- some kids just don’t do well on it, and we don’t know why. Some really do though, so we mustn’t dismiss it either.
The results do raise a question, however. If drug companies spent the money they lavish on marketing ADHD drugs on research, maybe the evidence wouldn’t be poor?