Most of the story took me in. A lot of the tension came through the relentless brutality that Wayne's character displayed to everybody. The development of the story let me feel the rising madness, paranoia and control obsession of the leader of the trail. There were two or three historical hints, but in general it felt pretty convincing as a story in itself. On a sidenote: I have never seen written text in a movie presented so fast - I could barely follow the text on the page whenever excerpts of the diary were presented.
The cast was perfectly chosen and the drift between Clift and Wayne was deepened by the knowledge at how far those two actors were in real life - in pretty much every sense.
One or two things bothered me slightly: The "other" gun hero and Clift never get that showdown that the story promises in the beginning. There is some rivalry between the two and it even gets noted by the other characters, there is a woman they could over, but ... nothing happened. There should at least have been some alienation between the two.
Also, the ending felt extremely sudden and slapped onto the rest. That was quite a let down and I wonder if the studio insisted on this type of ending (or the stars). Might be interesting to study the making-of, especially trying to find out if Hawks wanted to go into another direction with the ending.