This isn´t really an adaptation
I mean it follows more or less the plot, and takes the characters from the book but changes something more important: its tone and purpose. The book is serious (though satiric) and this movie is not. I could understand it if it was an outright parody but it isn´t that either. To me, the effect was a little irritating, and I think the low rating reflects that it happened the same to a lot of people. If you haven´t read the book I guess it is not so bad.
By the way, I can recognize "Heart of Darkness" in "Apocalypse Now" or "Macbeth" in "Throne of Blood", I give a lot of room to directors to change things but I think there is a limit (at least for the movie to be considered an adaptation).