My instructor at school was talking about this topic this week. According to studies he's seen, caffeine is not more effective than a placebo for ADHD. So essentially, it's ineffective.
But if people "think" that they can concentrate better with a cup of coffee, then I don't see any problem in that. But I don't think it can replace an attention deficit disorder medication.
I personally don't see a difference in my ADHD when I have caffeine. Too much caffeine has a bad effect on me, it gives me anxiety and heart palpitations.