This is a very highly rated movie which I felt just about lived up to its rating. Dean Martin is excellent and the chemistry between John Wayne, Dean Martin and Ricky Nelson is intriguing. There is a young and very beautiful Angie Dickinson who falls for Wayne instead of the younger stars! The plot is fairly predictable but Walter Brennan as Stumpy introduces the necessary comedy at times - the baddies are bad enough to deserve what they get but not so bad that they spoil the overall good feeling around the movie. John Wayne shows his usual grit to great effect.
This was made as a riposte to High Noon which Howard Hawks and John Wayne felt was anti-American, and they mocked the film for the notion that a sheriff would expect the support of his community; he should go out and shoot the bad guys alone. Though, in Rio Bravo, when the outlaws hit town, Duke has enough deputies and allies to fill a minibus.
The western it most imitates is Gunfight at the OK Corral, particularly for the comedy of its odd couple bromance. Wayne is the steadfast, sharpshooting Sheriff. Dean Martin is the charismatic, drunken deputy. The films share many similar details, but Rio Bravo is a more comic, cartoonish film. The characters have names like Stumpy and Dude. Angie Dickinson wears feathers and so is called Feathers.
And by the finale, Walter Brennan is throwing sticks of dynamite around like it's Looney Tunes. After Hawks made The Big Sleep he decided that the audience didn't care about the narrative, just the comedy and characters. Leigh Brackett wrote both films, and Rio Bravo is a series of archetypal western situations set into a loose narrative. The plot barely matters.
It's a long, episodic film and by the time Dino and Ricky Nelson present a couple of Mariachi ballads, it begins to feel more like a revue. There are a pair of comedy Mexicans. Dickinson reprises the Lauren Bacall persona from earlier Hawks films. But, personally, if it was intended to support the human rights abuses of the McCarthy witch hunts, then that makes liking Rio Bravo a lot to ask.