I think of it as basically stealing energy or enthusiasm from the future, though. You might feel energized and focused now, but it comes at the expense of less energy and focus when the drug wears off. The withdrawal effect is pretty mild if you take prescription doses of it though, e.g. Adderall (which is amphetamine). At normal moderate doses, taken in the morning, almost all of that energy can be recouped during sleep (though not all). I wouldn't want to take it daily for a long period of time though, otherwise you'll build up an 'energy deficit' that could lead to a crash.

P.S. I know people who have essentially destroyed their lives by becoming addicted to amphetamine or meth. It's a dangerous drug.

If you don't, it's negatively impacting mental performance - but it's really good at helping you to deliver slightly subpar performance for grossly extended periods of time. (Common uses: Crisis situations where you cannot step away. Cramming for exams)

I suppose you could call this a different form of concentration, but AIUI it's more energy than concentration.

If a person already has good levels and healthy receptors, and they suddenly raise them too much, it just makes people obsessive and actually not focus well on the important task at hand.

It's a double-edged sword, always with warnings and side-effects.