Pros:
Will Smith did a much better job portraying Ali than I thought he would given that he looks nothing like him and weighs so much less, (even with the weight he added for the role).
Cons:
Short on dialogue and looong on music that had no bearing on, or relationship to, the story other than it was in the same time period.
Continually jumped periods of time with no warning or explanation, (I ended up stopping the film several times to Google things so I'd know what year we were in.) which left holes in the story. (At one point Howard Cosell says "You know I'd do anything for you". Really? I hadn't gotten that from the film at all.).
Slow moving, and not that engaging.
Some of the worst sound I've ever experienced. I could barely distinguish a word of the film without it being turned up full volume. And, of course, the music was very loud.
Of the 6 Michael Mann movies I have seen, this is by far the weakest (with the possible exception of Miami Vice. You could make an argument for either being the worst for different reasons.) and far below "Collateral", "Heat", and "Last of the Mohicans".
Was this a stream of consciousness experiment? I don't know, but I actually ended up liking Mohammed Ali less after seeing this film. I guess the flip side of being a charming, confident champion is being arrogant, pig-headed, and feeling like you have a monopoly on the truth/what's right. Seeing some of his interactions with Malcolm X, his wives, and his manager left me shaking my head. For someone with such a fragile ego, he showed zero sensitivity to the feelings of other people.
Overall:
See it for Will Smith's acting if you must. Otherwise, prepare to be underwhelmed.