I knew a couple in Vietnam. Others used it, but didn't go crazy. It's can be bad news, but you also hear the same stuff about crack cocaine. Do you recall the friendly fire incident in Afghanistan in which our aviators bombed Canadians by accident? Their defense, which was insufficient, was they had taken dexamphetamine which was issued to them because of their extremely long range missions.
Both of those chemicals and their variants engage the pathways of the nervous system that use the same neurotransmitter, dopamine, just like morphine, marijuana, ethyl alcohol and nicotine.
Alcohol does to your liver what meth does to your face, so people don't see the ill effects of the former on a superficial basis.