I can see why this movie has not done so well in the theaters, and might not appeal to the mass audiences. The movie, from the very beginning does not fit the traditional "western" theme that many have learned to love from Hollywood. I don't think this was what Kevin Costner was looking for.
From the very beginning, you see the movie through many different lenses. It is raw, honest and lacks the sexiness of the good vs evil narrative that is so typical. I think he does an amazing job at depicting the life and struggles of that era especially those of the Native American People.
I really enjoyed the movie and history buffs will as well. Don't expect the story to be told from the typical point of view, but rather sit back and immerse yourself in what I think is its original honest intent. If you do you won't be disappointed.