A flawed masterpiece, they said....I suppose it's true. The difficulties and compromises in the making of the movie were well-known before Coppola aired them in the Hearts of Darkness documentary - even before AN was released. While the film may not have been the full realization of Coppola's vision, it remains nonetheless a masterpiece.
Many objected to Brando as Kurtz. They said he was bloated, unfocused, distracting. My feeling as he emerged from the shadows for the first time, half-lit and sweating, was that he did indeed contrast starkly with the rest of the picture up to that point, and that this made him appropriately fearsome. He could not be what you expected and have as much impact.
Some felt the voice-over narration was hokey and cheap, and in fact it doesn't look good on paper at all. I felt it added something, that it contributed to the noirish quality of the film. One time years ago I was in a record store where they were playing the soundtrack, which included some of the narration, and apart from the visuals it sounded a bit ridiculous. But with them, it works.
I've known several Vietnam vets in my time. One told me that seeing AN was like being there all over again, that it captured the experience perfectly. Another was of the opinion that the movie was nothing but trash. Me, I was never there, so on a purely personal level it could not be a "Vietnam movie" at all. What it was for me was a stunning confrontation between two ways of life: the Passive (floating downstream, not really in control) and the Active (Apocalypse Now!). It achieves, miraculously, a marriage between this interior dialogue about one's direction in life and a story with incredible cinematic sweep and beauty. The flaws are pretty easy to ignore.