The Magnificent Ambersons (based on the novel by Booth Tarkington) is the story of an arrogant young man who finally gets his "comeuppance." There were certainly people in Hollywood who viewed Welles as an arrogant young man and were glad that he got slapped down a little, too, when his second film was taken away from him while he was out of the country. The entire story is too long to get into here (see the reference books), but the last 45 minutes of Welles' original version was cut and a new ending was tacked on instead. The original footage is now lost (I've heard that the laser disc version ends with storyboards and script excerpts for the ending Welles originally shot).
The Magnificent Ambersons is the only movie Welles ever directed in which he did not appear. He had played George Minafer in a radio adaptation of the book a few years previously, but even with the girdle he wore in Citizen Kane he couldn't play the role on screen. He said later that he had really enjoyed directing without having to act in the movie as well, but I suspect that in some of his movies it was a commercial decision as well as an artistic one that he should do both. He may not have been a commercial draw as a director but he certainly was as an actor, plus (as he always pointed out) he was the only really good actor he could ever get for free. Spike Lee has also said that he would like not to have to be in all of his movies, but it would be much harder for him to sell a movie if he wasn't going to appear in it.
In any case, Tim Holt is better in the role than Welles could have been, so it all worked out for the best anyway.
Even with the butchered ending, anybody who cares about movies should see this film, and probably more than once. The tone is more reflective than many of Welles pictures, and it's more a portrait of a whole time and place and way of life than of a single man, as many of his other pictures are. The cast is excellent (still mainly drawn from his Mercury Theater actors he brought to Hollywood with him). Anybody who thinks of Agnes Moorehead exclusively as Endora on Bewitched should see this.
Welles narrates the movie, and the whole first section is devoted to vanished small-town customs and changing fashions, before any of the main characters are even introduced. Apparently this was the first movie to use narration so extensively. But I don't think the reason to go see Welles' movies is because they were revolutionary at the time. The reason to see them is because they are amazingly good.