I can drink 2 or 3 energy drinks in a row (at 160mg caffeine each) and not feel a thing. It might help me stay awake if I'm really tired, but I'm not even sure if it does that (and it certainly doesn't mitigate the sleepiness). I'm similarly insensitive to a lot of other chemicals too though - I am relatively insensitive to alcohol (I don't feel it until I'm most of the way through 12oz of a 10% stout, for example), and when I had my wisdom teeth removed, I needed to have significantly more anesthetic than the dentist had originally estimated. I do still drink caffeine from time to time, but it's largely just in the hope that it will help me stay awake (even if only slightly), since I don't notice a significant effect.