Leaving aside the 'classic masterpiece'/etc talk: a rough, realistic, humanistic and ethnographic depiction of life in the favelas of Mexico city. Great! Yet, I've found the use of music a bit exorbitant - and all this symbolism not really necessary -> such traits are congenital to a whole era of course, but I was born 30 years later so I don't really have to comply with them..
I prefer Buñuel's contributions to surrealism, but there's no denying that his depiction of extreme poverty is excellent. Sexual references mingle with loss of innocence, and the slums are like a different reality altogether. Buñuel himself didn't consider this as a neorealist film, and it's easy to see why, because his film has the kind of cynicism and surreal atmosphere that is lacking from the Italian genre.
Arguably the best film to emerge from the wave of post-war realism...I prefer it to anything I've seen from the Italians at the time. This makes it strange that it comes from one of cinema's most famous surrealists, or maybe not strange at all—it's great because it knows that dreams and irony are a big part of reality. One of Bunuel's best, and it adds a thrilling context to some of his more explicitly arty films.