There were a couple things which motivated me to finally catch up with it...
Christopher Frayling, a critic and writer I admire tremendously has written a huge, beautiful and definitive new book about the film.
And of course Quentin Tarantino's latest movie, Once Upon a Time in Hollywood is in many ways a homage to Leone's picture.
In 1966 Sergio Leone had just had three hits in a row with his ground breaking and earthshaking series of movies with Clint Eastwood (A Fistful of Dollars, For a Few Dollars More and The Good, the Bad and the Ugly).
Understandably, he was keen to leave westerns behind and make a different kind of film. What he had in mind was a gangster epic set in New York. (This would eventually become Once Upon a Time in America in 1984.)
But the American studio Paramount, while keen to work with Leone, preferred that he do one more western, expecting another box office bonanza.
What they got instead was an utterly different kind of movie, one which reinvented the western.
It is long (nearly three hours), slow moving, and almost bereft of dialogue. I would compare it to a great movie of the silent era, except its music is so important.
The score by Ennio Morricone is devastating. Morricone had collaborated with Leone from the beginning, and they had created three classic films with three unforgettable scores, but this time the men had the time and money to fulfil a dream.
Morricone's themes were written before shooting began and the music was played on the set to create a mood for the actors (something Quentin Tarantino still does).
And scenes in the film were created and edited to match Morricone's score, instead of the other way around.
Given all this, it's fascinating that the famous opening sequence features no music at all. In this three killers wait at an abandoned station for a late train, so they can ambush and kill Charles Bronson.
Morricone and Leone tried music here and it didn't work. So Morricone suggested instead that they use natural sounds — like the iconic squeaking windmill. It works fantastically well, and given that it's Morricone's concept, in an odd kind of way it is also his composition.
Once Upon a Time in the West was shot in Spain, Italy and Arizona at exactly the same time Sam Peckinpah was shooting another game-changing western in Mexico, The Wild Bunch.
But whereas The Wild Bunch was an immediate box office success, Leone's movie, released in America in 1968 in a shortened version, was a financial disaster there and in other English speaking territories.
However, it was a big hit in France, Germany and Spain. And in the years since its release it has come to be acknowledged as a classic.
Interestingly, Stanley Kubrick, another maker of long, slow films was a great admirer of Once Upon a Time in the West.
And of course Tarantino loves it: "it really illustrated what a director could do with the medium."
Leone's movie is incredible visually, with inspired set design and costumes by Carlo Simi and luminous photography by Tonino Delli Colli which balances long shots of landscapes with immense close ups of faces.
And what faces they are — the granite countenance of Charles Bronson, who would become a star as a result of this film, the feral beauty of Claudia Cardinale, and the cold blue eyes of Henry Fonda.
Fonda, who for decades had been a symbol of heroic decency in Hollywood films, is here reinvented by Leone as a profoundly evil killer.
If you have three hours or so to spare and a good wide screen television, I'd suggest having a look at a DVD of this film, or perhaps the new Blu-ray.
It is slow, sometimes silly (Kung Fu style sound effects for punches), has dubious sexual politics...
But it's a masterpiece.
(Image credits: the posters are all from the Movie Poster Shop. They have a great selection and deserve your business.)