Nick tells the story in a way that is, in one sense, unrealistic, but which is very satisfying.
The unrealistic elements of the narration grow out of Nick's telling of Gatsby's story. Yes, we are given a context that explains how Nick knows what he knows about Gatsby's past, yet this context is essentially a thinly veiled slight-of-hand on the part of Fitzgerald.
Beyond the formal oddity of using Nick as Gatsby's mouthpiece for extended space in the novel, I find Nick to be a thoroughly intriguing narrator.
He learns quite a bit and is sarcastic and intelligent enough to both let the reader know how much he is learning and to play down the importance of his epiphanies.