I actually take it myself. I see a lot of people take various prescription drugs on here ( search for Ativan or Xanax on the forums ) and I find it has a relaxing effect which would prove helpful on flights. Of course you should always consult your doctor, but Wikipedia has some things to say on the subject. I found the GNC formulation works best ( partially because it is made with a variant called suntheanine ).
Usually just one at night. Helps me sleep. On some days I'll take one upon waking if I have a stressful day ahead. And definitely one before a flight. It is something you'll need to experiment with as everyone is different and their reaction. Also I wouldn't save it just for a flight without knowing your reaction. I was quite foggy the first time I took it. Lastly. Always take on an empty stomach.