I'm currently taking GABA supplements (500mg and just GABA).
I had heard from several people that it wasn't good to take them for over 2 months straight (I have no idea why); but when I asked by my Pdoc she said it was okay, and that she had even put children on it, so it's not harmful.
My experience is that it helps me stay a little more calm, but the effect is not too big. There are no side effects that I know of (as long as you don't go over the recommended dose)