The first few times you take it or you go up a dose it feels pretty amazing. For some reason I want to say it feels like your head is full of light, but don't even know what that's supposed to mean. There's some extra energy and a little more motivation, but if you're looking for a pill that will turn you into a super-productive maniac look elsewhere. It mostly just keeps you awake and what little extra motivation you gain will be offset in part or in full by being more more easily distracted. I've never noticed any sort of increase in cognitive function. What I have noticed is that I get exhausted from physical effort very quickly.
When I take it long enough I start to pick off all of my facial hair (there was maybe a year where I never shaved, but had no facial hair), and lose a lot a weight quickly. Recently I dropped around 40 pounds in a few months.