I use it as a supplement because I do weightlifting. 5g/day. I did 20g/day for a week once, and didn't notice it made any difference, so I'm back to 5g now. In terms of stuff like memory, mood, etc. I can't say it's made any improvement but the idea that it might be helping prevent decline is nice.
Its changed my life. Im also a long time vegan with MTHFR gene mutations and sleep issues though.
For lifting you hit a saturation point quickly where your muscles will only hold so much of it. Not sure if there is an equivalent for any brain related effects.
I've found it helps cognitively if under a lot of stress and/or sleep deprived. If I'm not especially stressed and am well rested, I don't know that I notice any difference.