People have complained that the ending feels tacked on, comes out of nowhere etc, but in the book, the last chapter contains the same ending as the film, but is told over only a few pages, so the ending of the film is in keeping with the ending of the book.
I thought it made a bigger difference in the film because we were merely told it. It wasn't shown. That made it less credible.
As to which version I believe: it seemed to me that the early parts of the film emphasised not having to choose. He believed in multiple religions, he never said goodbye to his girlfriend, he did not close off possibilities. So I suppose we're supposed to believe both versions of the story. (Or neither: in real life, neither happened.)
The bit with the tiger was implausible, but the bit with the floating island impossible. When I was reading the book, I thought the narrator had got sick and started hallucinating at that point.