A much debated question! I remember that when I was studying English at University Achebe came and gave a lecture about this novel and how "overtly racist" it was. His main argument was that by choosing Africa as the site of a novel that explores the human condition and the abuse of power in colonialism, Conrad is committing another abuse of power at the expense of Africa and Africans. Personally, I don't agree with him. If you read this novel carefully it is possible to identify a distinctly anti-colonial agenda.
However, the critics mentioned above would no doubt want to comment greatly on the presentation of Africans suffering the abuses of European colonialism, such as in the "grove of death" where dying Africans are described as nothing more than a bunch of "angles." They would also want to discuss the attitude of the colonialists towards the Africans and the way that they are "othered" (to use Said's vocabulary) into positions of ignorance, weakness and stupidity. The women in the novel are particularly interesting examples of this, especially the aunt who talks of "weaning those ignorant millions from their horrid ways."
And yet it is important to remember that the novel presents us with a series of colonial approaches. There are those, like Marlow's aunt, who have been "taken in" by what Marlow refers to as the "rot" of the big idea behind colonialism. Others see colonialism strictly as something they can use to enrich themselves. Marlow is very open about presenting alternative and critical approaches towards colonialism. Consider the following quote:
The conquest of the earth, which mostly means the taking it away from those who have a different complexion or slightly flatter noeses than ourselves, is not a pretty thing when you look into it too much.
The only thing that "redeems" it is the "idea," which is an idea that you can "set up, and bow down before, and offer a sacrifice to..." This is rather a worrying statement that foreshadows the kind of devotion that Kurtz expresses and the lengths to which he goes to "offer sacrifice to" the idea of colonialism. Bahabha would make much of this comment as it points towards "the essential ambiguity of colonialism." On the one hand we have the "idea" that justifies the colonial endeavour, and yet the novel points towards the way that this "idea" is used and abused to disempower the Africans, in contrast with the idea that is supposed to help them.