Marlow narrates the main story inside the narration of an unnamed narrator, who offers few opinions and serves to represent the reader. Marlow's narration is often obscure, full of philosophical digressions and his opinions of the people and places he encounters in the jungle. While he cannot be said, in any sense, to be an objective narrator, he is capable of very memorable imagery and insight.
I saw him extend his short flipper of an arm for a gesture that took in the forest, the creek, the mud, the river,-- seemed to beckon with a dishonoring flourish before the sunlit face of the land a treacherous appeal to the lurking death, to the hidden evil, to the profound darkness of its heart.
(Conrad, Heart of Darkness, gutenberg.org)
Marlow does tend to get bogged down in explaining his internal mental struggles, and goes off on tangents; sometimes, the pure narrative of the story gets lost in his digressions. However, he keeps the story moving through the sheer horror of the things he has experienced. Joseph Conrad's writing style will appeal to some and not to others, but he has crafted a very deliberate character with Marlow, a narrator with a specific and individual voice and style. In this sense, Marlow is an authentic narrator; the "goodness" of his narration is left up to the reader.