This movie was impressive because it followed the 'sports movie' formula just enough to capture a wide audience, without using but a few of the staple formula elements. Of course, it helps when the subject your movie is the real life core material through which many sports movies are made.
Mohommed Ali is a very colorful character, but the film primarily focuses on the time period in his life between his first aquisition of the world championship, the circumstances that surrounded his loss of it, and his reclaiming of the second. Initially we got the movie thinking it would deal about the tragic experiences later in his life, but that does not happen in the movie. Instead, it deals with the harsh hand that he is dealt, and how he overcomes them.
In short, it explains how he became the icon that he is and why. It is a story of overcoming absolutely overwhelming odds, that also happens to be true. Finally, it does so in a way that does not have a sappy artificially sachrine nature to it. That's just not the nature of Ali and as such it is not the nature of the movie. Will Smith proves (yet again) that he is an actor beyond par in this, nailing Ali's mannerisms in ways that are quite impressive.
You also get a whole new respect for Howard Cosell besides a voice that is difficult to forget. Very good movie.