I noticed an effect when taking it with coffee, too. I just had some l-theanine as a supplement. I just drink coffee, so I didn't think much of taking it with coffee, so I didn't expect anything. It seemed to "smooth out" the effect of the caffeine. Rather than jittery, it turned it into smoother focus.
Just throwing my own anecdotal experience on to the pile: L-Theanine is very effective for me when taken with caffeine in a 2-1 ratio. As another commenter said, it "smooths out" the jitteriness of caffeine, and this scales up to 200-300mg of caffeine (400mg-600mg of L-Theanine). The focus it provides is the closest legal alternative to adderall that I've found.
If taken alone, it's also pretty good at providing a general reduction in anxiety. I was travelling internationally one time with a co-worker who had a very low grade anxiety attack. Due to the circumstances our options for getting him medication or seeing an actual doctor were limited. I offered him some L-Theanine and he LOVED it, he said it calmed him down just enough the rest of the trip that he wasn't concerned with having another attack. Obviously, this isn't medical advice and ideally he would have seen an actual doctor, but in this case it was really effective. I will say that taking it without caffeine comes with a bit of sleepyness. Definitely a potent combo when taken with Melatonin and a lot of OTC sleep aids will combine the two.
Caffeine also “treats” things like ADHD, sometimes without someone realizing it. I was told I likely had ADHD as a child that wasn’t ever treated/evaluated (because hippie parents) that carried into adulthood and I have basically been self-medicating with caffeine for the last 10+ years.
He's not advocating for the brand "Bulletproof", the coffee recipe itself is simply called "bulletproof coffee", it's the combination of normal coffee (whatever type you choose) combined with MCT oil and Ghee/butter.
I purchased the powder on amazon (Nutricost L-Theanine Powder). It comes with a 200MG scoop and I just mix that into my cup of coffee. For me, it lasts around 2 -3 hours, doesn't impact the taste, and eliminates the jitters that I receive from caffeine.