Michael Mann's Ali is apparently a good film for some people. But it's not for me. I suppose most fans of the movie are amongst the fans of Muhammad Ali / Cassius Clay. The usual reason for such appreciation is fans of a particular person or topic are more inclined to like stories about that person or subject. But in this case there is something more: director Michael Mann has chosen to tell the story of Ali in a way only people which have enough knowledge about the boxing legend are able to comprehend and appreciate his movie. Why Mann has chosen to make the film like that is beyond my understanding. What I know is I generally enjoy movies about men like Muhammad Ali but gathering a bunch of random facts about someone and basing otherwise unrelated cinematic sequences on these facts is one of the least successful ways to entertain me.
As expected, Ali is a biographical film about sports legend Muhammad Ali. Various moments from his early life and career are displayed and even a completely unfamiliar spectator will understand that Ali has been a great boxing champion and a very interesting man. The problem is it's hard to understand anything more substantial from the movie. There are plenty of Cassius Clay's life aspects referred in Ali, being either his boxing success or his religious life or his years in "disgrace" or his trip to Africa. But everything is treated in a way if it is told to people who already know Ali quite well. It is a strange approach to tell a story, especially of someone whose glory days have been a couple of decades in the past.
The movie is not void of good acting. Will Smith as Cassius Clay / Muhammad Ali and Jon Voight as Howard Cosell are particularly notable but the rest of the cast deserves good words too. There are also a few enjoyable cinematographic moments thanks to cinematographer Emmanuel Lubezki. And there could have been an interesting screenplay. But in reality the writing is not good. The lack of a coherent story makes the film very diluted. "Creative" ideas like following Muhammad Ali's "running" in Africa for a good number of minutes doesn't help either. The dilution of the material is so strong there are some presumably important characters in the film, which the unfamiliar spectator will still stay unfamiliar with until the very end of the movie. It's hard to follow the narrative because there is hardly any narrative to be followed.
Saying all of the above, I can see why there are people who vastly enjoy Ali. If you've been a fan of his through the years and you know a lot about him, it should be really cool to watch sequences reminding of events you've personally experienced or you have read about in your earlier life. I suppose it is comparable to the experience of seeing a series of brief moments of your own life (like on a film tape) in an existence-threatening situation with the difference the life is not yours in this case. But for others, not having a decent knowledge of the legend, watching the movie is more of a painful adventure than a pleasant trip. And just in case, you can always watch Spring Breakers if you want to feel better about Ali.