IMDb: For the viewer who comes upon it long after its making, "Winchester '73" has something in common with "Casablanca." While you watch it, you get this feeling that you're looking at a string of clichés encountered so often in the genre; then you realise that the clichés became clichés only after being copied from this particular film, and that they were so widely copied because this film was so great. In other words, it's a seminal work.
"Winchester '73" is a joy to watch. The broad lines of the plot are somewhat predictable, but mostly because you've seen them copied so many times in later movies, and nevertheless it still contains a number of twists which surprise you. The dialogue, the pacing and Mann's direction are excellent. Stewart shines in particular, and if you're a fan this is a "must-see," but he is not alone in delivering a good performance. Remarkably, many of the most thoughtful and/or witty lines go to minor characters. Because this makes these characters (much) more than cardboard cutouts, it lent additional realism to the film.
This is a remarkably underrated film, and well worth keeping an eye out for. The DVD also contains an interview with Stewart which provides some background on the film.