The positive, if you want, could be the fact that everything that was hard for him at first, turns to be easier. The world itself turns out to be not so complicated.
In turn, teh negative side is that everythign he liked, well, gets him bored now. Think how the relationship with her changes, and turns from nothing...to something... and then he feels superior, she just cannot keep the pace with heim because he is SO smart now.
About Charlie's feeling, he's always confused I think. Even though he understands what he is going through, well, xwhen the drug stops working, all the confusion and enigma of the world comes back.
Hope it helps