I've had some really hellish experences with amphetamines when i was perscribed them for ADD as a child. Recently, i tried taking some of my old medication, and described it to a friend as if my soul had been torn from my body, not really a very pleasant experence.
I'm also not a big fan of opiates, i seem to be unusally sensitive to them, and whenever the intial hit starts, the room starts spinning around me, not something i enjoy.