There is so much to like and appreciate about this film that while it's an enjoyable experience, it isn't the great film it should have been.
Firstly, the script is largely excellent. It has a good plot and characters backed up by interesting dialogue. It has a top-notch cast delivering almost universally quality performances. As well, it has some interesting themes and issues to explore, especially in the central battle between Fonda's warden and Douglas' prisoner. The scene where Douglas confronts Fonda in the just built eating hall and exposes his self-serving interests and hypocrisy, is a great example of top-class screen writing.
All the elements are there for a classic (or at least semi-classic) Western, but it doesn't quite reach that. Why? I think a big problem for this is Joseph L. Mankiewicz's direction. As other users have commented, the tone of the film is jerky and erratic and he has to take prime blame for that. But even in pure cinematic terms, it isn't well directed. Scenes that should have been highlights (such as the robbery that opens the film) lack punch because they're ineptly handled.
I think another major problem is the cinematography. The glossy, bright and flashy look of TWACM seems more in tune for a jovial, knockabout, straightforward Western. For a film full of cynicism, complexity (as well as its share of humour) and some rather depressing elements, it's a distracting and misjudged look. The much more realistic style that was to become much more common in films as the 1970s progressed (e.g. 'McCabe and Mrs. Miller') would have been much more apt.
Overall, an under-appreciated and underrated film worth seeking out. But also a bit of a missed opportunity.
