Despite the crap the filmmakers had to endure to get this one done, it took its share of pans when it came out: A pious piece of agitprop full of too-good-to-be-true and too-bad-to-be-believed stick figures, etc etc. Today, it holds up well - first, its use of "real" locations and "real" people appears more valuable in a documentary sense the farther away we get from the time it was made. Second, the production values, especially the cinematography - the Blacklist claimed some of the more talented technicians in Hollywood, and Salt of the Earth benefits richly from their work.
Third, the themes remain quite relevant. When we see footage of, say Bolivian coca growers taking to the streets to overthrow their country's US-sponsored tycoon president, what's so surprising about a community of Mexican American workers standing in solidarity against an exploitative mining company? When we see Justice for Janitors bringing the owners of LA's office towers to the table (at least), what's so far-fetched about workers in Salt of the Earth grabbing a bit of justice for themselves? I could go on.
From the vantage point of 2003, Salt of the Earth looks like a refreshing change. Agitprop is news to a lot of people today - it can be powerful if done well, yet we're now all conditioned to think that any form of dramatic art that doesn't center obsessively on the isolated individual is false and/or sentimental. Is Salt of the Earth really more sentimental than On the Waterfront (made about the same time), in which a corruption struggle on the New Jersey docks serves merely as the scenery for Marlon Brando's emoting about his boxing career?? Come on!!
People who stand in solidarity really are powerful. Americans are taught not to think so, but it's when they stand up together, not separately, that they win the biggest victories (and I don't mean in uniform, either).