If you want something that will allow you to be closer to your full potential, I recommend these.
About 2 weeks ago, I went a full week without them because I was in Florida. Immediately noticed a difference. I didn't feel nearly as sharp as usual. Got back home, resumed taking them daily and noticed the difference right away.
I've been taking it for over a month. It's always made a noticeable difference. I'm always more alert, and more able to think quickly and clearly even when I don't get enough sleep (which is often).