Quote from Okami;255376:
I honestly can say I don't think I've ever crashed from one :/
I barely get effected by them at all.
I know people who go absolutely crazy from the smallest bit of caffeine. For me, I need a full blast to really make a difference, and even then it only serves to keep me awake on a really tired day.
And then several hours after, I go to sleep anyways.
Basically I use it as an emergency wake me up, for when I spent the whole night studying for a test and I'm too tired to actually take the test. Something like that.