My first encounter with The Three Musketeers was the 1993 Disney movie, which I adored and can still mostly recite, I watched it so many times. I was thrilled to finally have my own copy of the book and devoured it. I was sorely disappointed with what I found, for the movie drastically simplified the plot and made virtually all the characters (Milady particularly) more sympathetic.
King Louis is jealous of the Duke of Buckingham because he believes that Queen Anne is endeavouring to conduct an affair with him. Cardinal Richelieu is jealous of Buckingham because he is in love with the Queen. d'Artagnan's father was not a musketeer. Rochefort is not completely evil and, actually, turns out to be fairly decent despite being one of the Cardinal's men. The Cardinal himself is not evil. The musketeers are not disbanded. Milady is evil, extremely evil. Athos tried to hang her and the two believe each other to be dead. Lord de Winter prevents Milady from reaching Buckingham. Buckingham is assassinated (part of the Cardinal's plan that Milady undertook in case she could not convince him to end the war, which she had no chance to perform because of her brother-in-law). Constance is married and not a lady-in-waiting. She is murdered by Milady. Milady's history is far more heinous than the movie would lead you to believe. The Cardinal is not trying to depose the King. Aramis wishes to enter the (Catholic) Church, Porthos is a buffoon. Buckingham is not planning to invade La Rochelle, he's planning to relieve the Protestants that are under siege from the French Catholics. There are a lot of other characters, and a lot more complications.
On this second reading, I enjoyed the book more. I do not particularly like the style of writing, but I suspect that's an artefact of the translation rather than Dumas, as I have not had the same problem with other books of his. It's an intriguing story, exceedingly complex, with rich characters. How faithfully the book captures the personalities of the people that actually existed (Cardinal Richelieu, Anne of Austria, King Louis XIII, the Duke of Buckingham) I do not know, but it makes them interesting characters that take part believably. None of the main characters (with the possible exception of Milady) is purely good or evil. All are flawed in some way. All are complex and believable, even the curious character that is Milady.
This is a book that's worth reading, though it may need to be read multiple times to appreciate all the detail.