This film made a huge impression on me when I first saw it at the age of 15 or 16. A recent rewatching on DVD really served to bring home for me what makes this film so special.
The whole thing is quite good, but it really hits you when Howard goes off to celebrate with the Indians, leaving Dobbs and Curtin to care for his gold and burros. The ensuing scenes of their spiraling mistrust and tension are absolutely spellbinding--the kind of thing that makes you lean forward in your seat just to get your eyes a little closer to the raw humanity unfolding in front of you. Their paranoia, the way you can SEE scenarios of betrayal dancing in their eyes, Dobbs' burgeoning madness--these are the moments that make this film one for the ages.
At its best, film noir (which this most certainly is--Western surroundings or no) makes the viewer complicit in the evil depicted on screen. We find ourselves scheming and plotting in our heads along with the unsavory characters we are watching--we start to feel the same temptations and desires that they do. "Treasure of the Sierra Madre" accomplishes this bond with the audience as well as any film you are likely to see.
A magnificent film--one of the few great screen tragedies.