Is Joseph Conrad's "Heart of Darkness" anything like Apocalypse Now which was based on it?

I'm thinking I would really like to read Heart of Darkness because of how much I enjoyed Apocalypse Now and the themes behind it. Is HOD actually similar to it in the themes of human evil etc? Is it as hard-hitting because I heard someone say the other day "it portrays Africa" as if that's its only purpose? Is it an interesting book?