I read the book first then watch the movie, and both are great!
The book of course is more detailed than the movie but the movie portrays the characters, the spirit, the epic moment very well.
My first Matsyuda Ryuhei's movie and he does not disappoint. He acts as calm dedicated nerdy Majime so well, so very well that I am actually sad that the movie is only 2 hours long. It should be longer, covering more detailed scenes with him working on The Great Passage.
Overall, highly recommended.
If you are comfortable with reading, suggest read the book first then watch the movie - or.... vice versa. It doesn't matter. Both are great (I said that again, didn't I) ^^