(ouch, too long ago)
I liked the story, though some parts couldn't "catch me".
There was darkness in the writing, and I could easily imagined the darkness of that world, though whether he described it so perfectly, or whether I overlay my own dark times in days of travel, I cannot say.
update April 2014
Read it again while in Africa. Big change. Many sentences and words got to me.
"We were cut off from the comprehension of our surroundings; we glided past like phantoms, wondering and secretly appalled, as sane men would be before an enthusiastic outbreak in a madhouse. We could not understand, because we were too far and could not remember, because we were traveling in the night of first ages, of those ages that are gone, leaving hardly a sign and no memories."
"Well, you know, that was the worst of it - this suspicion of their not being inhuman. It would come slowly to one. They howled, and leaped, and spun, and made horrid faces; but what thrilled you was just the thought of their humanity - like yours - the thought of your remote kinship with this wild and passionate uproar."
"You can't breathe dead hippo waking, sleeping, and eating, and at the same time keep your precarious grip on existence."
"They only showed that Mr. Kurtz lacked restraint in the gratification of his various lusts, that there was something wanting in him - some small matter which, when the pressing need arose, could not be found under his magnificent eloquence. Whether he knew of this deficiency himself I can't say. I think the knowledge came to him at last - only at the very last. But the wilderness had found him out early, and had taken on him a terrible vengeance for the fantastic invasion. I think it had whispered to him things about himself which he did not know, things of which he had no conception till he took counsel with this great solitude - and the whisper had proved irresistibly fascinating. It echoed loudly within him because he was hollow at the core... "
"but it was something to have at least a choice of nightmares."
"It is his extremity that I seem to have lived through. True, he had made that last stride, he had stepped over the edge, while I had been permitted to draw back my hesitating foot. And perhaps in this is the whole difference: perhaps all the wisdom, and all truth, and all sincerity, are just compressed into that inappreciable moment of time in which we step over the threshold of the invisible."
"I found myself back in the sepulchral city resenting the sight of people hurrying through the streets to filch a little money from each other, to devour their infamous cookery, to gulp their unwholesome beer, to dream their insignificant and silly dreams. They trespassed upon my thoughts. They were intruders whose knowledge of life was to me an irritating pretense, because I felt so sure they could not possibly know the things I knew. Their bearing, which was simply the bearing of commonplace individuals going about their business in the assurance of perfect safety, was offensive to me like the outrageous flauntings of folly in the face of a danger it is unable to comprehend. I had no particular desire to enlighten them, but I had some difficulty in restraining myself from laughing in their faces, so full of stupid importance."
update 2019-07-22. Read the last part again when coming across this book in the grey house in Scotland, 6 miles south of Ullapool. The exact same part - "I found myself back in the sepulchral city" - spoke to me as strongly as it did then.