Yeah holy shit, I would not recommend parachuting 90 mgs of Adderall for someone who's naïve to the drug. You'll definitely have a blast if you're inclined to enjoy it, but you could have a bad reaction and not enjoy it. And at that dosage you'll be strapped in for the ride LOL.
As far as what it feels like? Dextroamphetamine for me just improves wakefulness and gives (very mild) euphoric effects. It'll keep you awake for hours and hours and hours, unlike the relatively short duration of caffeine, and without the jittery effects of caffeine. It also has an anxiolytic effect (for me, anyway) and improves my motivation to do things. "You need to change the oil in your car? Fuck yeah dude let's go out there and do it RIGHT NOW!" A lot of times at high doses it seems like everything is moving in fast forward to me...it's especially crazy if I'm walking through a crowded place, like a shopping mall or something. But, at the same time, the comedown from something like Adderall is really mild; the come up isn't that intense, but neither is the come down, so it's pretty nice and functional. I can go to sleep on it pretty easily, if I feel like it, and my appetite is relatively normal.
Methamphetamine, on the other hand....*grimaces*