Yes. I took ten grams at once. It wasn't bad; an hour later I felt incredibly relaxed, like I was just the teeniest bit high, but not much other than that. Not the cheapest habit to maintain, but it was a good one-time experiment.
This is the highest dose of L-theanine I've seen taken. At one dolar per 10 grams, It's actually reasonably affordable to take daily for medical purposes. I may try it, but I'm unlikely to implement it long term. If it's a significant effect, it would be a useful tool. However, I'd be concerned about tolerance.