I've seen it twice, as it gets better on repeat viewings.
There's glaring differences from the novel, such as the origins of some characters, the
Minutemen are only really given a mention and the whole of Tales Of The Black Freighter being absent, but as a whole it's pretty close to the source material.
Also, although most hardcore fan boys will shit a brick over it, the endgame is far better in the movie than it was in the novel.