I have read the Halo: The Fall of Reach book a few times and was actually reading it around the time I saw this movie.
Basically the movie follows the outline of book fairly well, but...
Somehow they managed remove the parts of the book that made it so good.
In the movie: John is kind of a dick, there isn't really any relationship between John and Chief Mendez (John is supposed to really look up to him and respect him), Captain Keyes is kind of a dick, the relationship between John and his fellow spartans is hollow. Everybody felt very 1-Dimensional. I think these problems are because of the lack of detail in the movie and the bland animation, the sometimes very off-key voice acting.
Honestly, if the people who made this movie would have just payed attention to why the characters in the book were so good and why the relationships were compelling we might have gotten something much better. That's too bad.
The book has great potential for a movie, but it was wasted with this attempt.
If you want some quick general backstory on Halo, watch this. If you want entertainment and a compelling story with compelling characters - read the book.