I thought it was a very good movie. I have never read the book.
I thought the movie was well done on a couple points:
1) the authenticity of the Norsemen's equipment, attitudes, way of life, etc. There weren't any unbelieveable characters.
2) Bandera's character, though the hero of the story, was not "the best" at everything the way many movie heros are made out to be - he did what he could as he tried to adjust to the Norse way of life and being thrown into an impossible battle against fiendish creatures.
3) I really liked the way they portrayed Banderas learning the language as he travelled with the Norsemen. No subtitles to benefit the viewer; we struggled right along with him, trying to figure out what they were talking about. But as he learned more and more of the language, more of the dialogue was spoken in English - it was a well-done, gradual transition.
Anyway, I gave this one a 4 out of 5. To me it was like watching a Dark Ages version of Saving Private Ryan.