Unlike professional critics, who don't write novels (I do), and don't understand adaptation, this film was a like a Bach fugue transposed from violin to guitar. I own and have read the novel, a great work of literature. It is not a bad film at all. The acing is astonishing. And the editing is fantastic. I instantly understood Theodore. You will remember how you were before seeing it, and how you changed. Good art challenges your world view, and this movie does make you question your assumptions.
Watch it. It is a magical experience about innocence and the world. If anything, it teaches us about regret, meaninglessness, and the redemptive power of kindness, loyalty, and love.
Ignore the critics. They get paid to be disenchanted. I don't get paid a penny for my opinion, and I say this is great art.