Muhammad Ali (formerly Cassius Clay), like many others, was an interesting persona because he pretty much did what he wanted and stuck to his principles, even if it was self-destructive. This is a great quality in a person in a world where the vast majority of the populace are quick to conform. Ali, the film, focuses on this aspect of the famous boxer and generally succeeds, even though it drags for too long.
Ali (played by Will Smith) was the person who fought against the U.S. government to avoid fighting for them, even though it meant losing his heavyweight championship title. He was the person who went against the Nation of Islam even though he had converted to Islam, and against Malcolm X even though he was against the systemic discrimination against blacks that was more common a few decades ago. More recently, he has fought Parkinson's disease with the strength he displayed as a boxer. (I use the past tense not because he is dead, but because his iconoclastic nature has become streamlined for general public consumption.)
Ali possessed a motormouth that he wasn't afraid to put to good use. This aspect of his character is what is most well-showcased in the film. Will Smith plays a convincing Ali, which I didn't think could be possible. It's easy to forget, while watching the film, that he was formerly the Fresh Prince of Bel Air. Smith's acting is nicely complemented by excellent performances from Jamie Foxx, who plays his trainer Bundini, and Jon Voight who plays Howard Cosell, who spars words with Ali in a way that indicates a sincere friendship.
The movie doesn't focus too much on the boxing (and even when it does, it's nothing exciting), which is good since boxing is a stupid, exploitative sport. Watching boxing is even more inane. It boggles my mind that issues like equality between races and sexes, and abridgement of human freedoms, are thought of in such ambiguous terms, whereas pounding someone in the face over and over in the name of sport and entertainment is much more acceptable. While Ali comes out on top on most of the fights depicted in the film, he took his share of punches later on, something that is not revealed in the film nor actively realised by most people who are familiar with his life.
The biggest problem with the film are the long extended sequences, with songs that aren't completely congruent (and I happen to like Sam Cooke), where nothing happens. Ali is a short-hand version, viewed from mainstream lenses, of why its namesake persona has earned his reputation. It is worth watching if you are a bit on the patient side.