When my son was diagnosed, our ped gave us all a great explanation of what ADD is and later when my ds wouldn't take them, why they help. She explained that his brain works differently than someone who doesn't have ADD. His brain goes so fast, it doesn't stop to make help make a good choice, or decide what needs to be done first. An ADD person sees a pile of work as an overwhelming pile, not something they can do one at a time. They are also more anxious and always need to be moving. So the medicine helps slow down his thoughts to help him make better choices, and focus on a task.
She was very clear with him that there was nothing wrong with him, his mind just worked way faster than ours :-). She also discussed kids with other chronic conditions and that they take meds and if they didn't, they wouldn't be better.
We sat down and discussed what he thought the meds helped with and discussed the choices he made when he was on and when he was off the meds. We talked about how he was successful in school/sports/activities on the meds but off, he was always behind or hindered by his "ants" (in the pants).
Ultimately seeing that he could get his work accomplished while in school, and not have homework and his success with his grades seems to have won out.
I also agree with the person who said you should take the meds consistently. When you don't take them you do get the side effects and it is more noticeable to them (headache/belly ache/anger). Once on them consistently the side effects are not noticeable.