So looking at the peer reviewed papers, it looks like:
From the abstract-
Studies examining caffeine's effects on cognitive, psychomotor, and affective functioning of children with ADHD were reviewed.
For children with ADHD, caffeine was more effective than no treatment in decreasing impulsivity, aggression, and parents' and teachers' perceptions of children's symptom severity, and more effective than placebo in decreasing hyperactivity and teachers' perceptions of children's symptom severity, and in improving executive functioning/planning.
Methylphenidate was more effective than caffeine in decreasing hyperactivity, aggression, and parents' and teachers' perceptions of children's symptom severity, and in improving executive functioning/planning and motor coordination.
Amphetamines were more effective than caffeine in reducing hyperactivity, impulsivity, aggression, and teachers' perceptions of children's symptom severity.
Combining caffeine and stimulants to produce moderate levels of arousal may produce better functioning than caffeine or stimulant drugs alone.
Research is needed regarding caffeine's effects on the functioning of adolescents and adults with ADHD.
Subscription and open access journals from SAGE Publishing, the world's leading independent academic publisher.
journals.sagepub.com
Also for adult AD/HD a study on soldiers found that:
The use of caffeinated compounds appears to be largely diffused among ADHD military soldiers and it could improve cognitive performance, despite the negative repercussions on sleep.
According to our data, in fact, caffeine may positively influence specific areas of memory and attention, but still the definition of its mechanism and target specificity remains unclear.
It also works on the rats they bred to serve as test subject to be the closest approximation of someone with AD/HD.
The spontaneously hypertensive rat (SHR) is generally considered to be a suitable genetic model for the study of attention deficit hyperactivity disorder (ADHD), since it displays hyperactivity, impulsivity, poorly sustained attention, and deficits in learning and memory processes. Converging evidence suggests a primary role of disturbance in the dopaminergic neurotransmission in ADHD patients and in SHR, and in addition, some studies have also demonstrated alterations in adenosinergic neurotransmission in SHR. In the present study, adult female Wistar (WIS) and SHR rats received caffeine (1-10 mg/kg i.p.) 30 min before training, immediately after training, or 30 min before a test session in the spatial version of the Morris water maze.
Abstract. The spontaneously hypertensive rat (SHR) is generally considered to be a suitable genetic model for the study of attention deficit hyperactivity
academic.oup.com
So basically it seems that caffeine works but really isn't the best possible treatment.