However, I am a big advocate in saying that you should never compare a movie back to its book, and I use this movie as an example. This story has been "Disney-fied" so that it can be called a family film. If you read the book, a true adaptation would not be family entertainment. Disney changed everything that they do. Read the Tarzan novel and compare to the cartoon. BIG changes there. The Little Mermaid. How convenient that Disney left out the fact that Ariel dies at the end of the story.
But what we should judge is the end result. This movie is still entertaining, despite having nothing to do with its literary influence. The characters are portrayed with the same attributes that they have in the book. For instance, D'Artagnan, while very duty bound and honorable, is young and headstrong, and prone to impulsive decisions that will help him to prove his skill and worth. Porthos is self serving and self praising, very vain and cocky, yet has a lust for the finer things in life. Aramis is humble and religious, but very skilled and intelligent, making him a very formidable soldier, yet he also loves the finer things in life. And Athos loves his wine, trying to bury himself in a alcoholic haze to hide the pain that he suffered in losing the love of his life. All of these come through in the movie, and all of the actors were great in performing them.
As far as the story is concerned, Disney likes things black and white, good vs evil. And so, the story changes to make the Cardinal a power hungry man with his own interests in mind. He wasn't like that in the book or in real life, but he was underhanded, and Tim Curry does another great job as the villain that he steals the show.
Overall, a great and enjoyable movie, worth watching with the family.