Most people don't need it since it's a "conditionally essential" AA. This means your bod does make some, but your need for it can exceed this production. The kicker is that very few people work out hard enough and long enought to do this. I notice a difference when I'm doing 20 hours a week on the bike for early-season base, but that's about it.
At any rate, it's certainly not going to hurt you. Pro-Score is one brand I can think of offhand that contains l-Glutamine.