On the other hand, I haven't seen the film.
I just finished the book a few minutes ago. Apart from the inevitable ending of the story, I didn't feel it was overly bleak. In fact, hiding underneath the initial conflict and melancholy, there was a quiet sense of hope at the heart of it all.
I think this is gifted to book chiefly through the first-person narration by Bromden, which I gather was lost from the film (along with some key parts of his character development). So I can imagine that contributed to a bit of a shift in tone.