My mom is an elementary school teacher and she has heard of Brain Gym. She finds it quite useful in her class.
While we haven't done "Brain gym" specifically, we have been doing eurythmy with my son, which I believe is based on similar principles and uses similar techniques. I think it would best be described as movement therapy. My son has been described by a few doctors we saw as "textbook ADHD", and we chose not to medicate but are doing Eurythmy instead. IMO, it has helped him tremendously and I see very strong differences in him now than at the start of the school year. But then again, he's also 6 months older now

So who knows... I am a believer, but I don't have any empirical evidence to back it up. At the very least, it can't possibly HURT, and it *might* be helping, so that's enough for me to keep going. Plus -- DS likes it!