The word's thrown around a lot, but this is the only case where I've actually been literally haunted by a book. It surfaces in the mind when I don't intend for it to, and its disturbing. I've never been really psychologically messed with by a book like this. I mean, not bad, but I get chills thinking about it. Like someone said, the images are great. The ship firing into the continent is one of those I can't get out of my head. And for those of you who are curious after seeing Apocolypse Now, I think they really messed up the Kurtz character. Its one of, if not the one of, my favorite movies, but he's so overacted and the script never really tells what the horror is. From the movie I got that it was the situation and man's situation, but from the book it is definitely the mind and soul, in my opinion. Any social commentary in the book is secondary to the more philosophical and psychological, here, I feel.
I also enjoyed Lord Jim and every one of his other short stories I've read, but none have been as good as Heart of Darkness. I'll probably read Nostromo pretty soon, too. If you want another take on a somewhat similar situation in colonial Africa, check out Journey to the End of the Night by Celine. I didn't so much care for the book as a whole, but that part has stuck with me.