Knock-offs tend to make me crash (Note, I only use these when I'm working a 6 hour shift). After about two-three hours, most other shots leave me with a hot sensation around my face and an uneasy feeling in my stomach.
I have never had a negative experience with this stuff though. They know how to make it.
Also, "5 Hour Energy" is vague. It keeps me alert for all six hours of my shift... and some afterward... depending on how hard I work myself.