Too much makes me hyper. If I drink it much past noon, I may not sleep well that night. It affects different people differently. I limit mine as I enjoy a more relaxed way of living.
Like many addictive drugs, the more you take, the more it takes to get the affect up to a point. A friend of mine who normally doesn't get much almost hyperventillated when he got a big jolt.
The good news is that it does improve mental performance in moderate amounts and there seems to be no negative impact on health, again in moderate amounts.
Al