It is quite effective for me, but I suspect that I am a slow metabolizer because even a small amount (less than a cup of coffee) can still have lingering effects the next day.

However, it does not affect me quite like it affects most people. I find that caffeine before bed helps me quiet my thoughts and fall asleep, for example.

Mind you, I have not been diagnosed with ADHD (I can't be bothered going through the process of diagnosis when I already have medication and techniques that work).