The Heart of Darkness is a deconstruction of European colonialism. The character of Kurtz symbolizes the hollowness and hypocrisy of colonialism as a "light unto all nations," since he's revealed to be twisted, mad, and death-like. Before Marlowe meets him, however, he appears to be a Renaissance man who's talented at everything and just wants to "bring the light of civilization" to Africa. In reality, he's just doing it due to greed and egomania.
All in all, a great read. I also loved the theme of truth and perception. After meeting Kurtz and returning to Europe, Marlowe questions if it's possible to actually really know someone. Because everyone views Kurtz (and European colonialism, by extension) in such a positive light, he starts questioning whether he really knows the truth - about life, Kurtz, and colonialism.
As for the writing, it is somewhat bloated and difficult to read, but that's to be expected since this is a 19th-century novel and English was Conrad's third language (he only became fluent in English in his twenties)