Gary Oldman makes the whole movie
Well maybe not all of it, but a lot of it.
The movie was based on the book "The Westies." Book's better than the movie, but I really liked the movie. However, Gary Oldman is the one that makes the movie work. He's based on Micky Featherstone, who eventually turned on the Westies to the Feds. Ed Harris' character is based on Jimmy Coonan, the head of the Westies (close too, but unreleated to Oldman's character in real life) who eventually is seen as a sell out to the Italians by his neighborhood folks and crew.
Gary Oldman plays the part of dysfunctional but lovable psycho better than anyone in any mob movie. He flies off the handle better and just gives a better performance than anyone else has done in the genre. This movie also has the best mob movie fight scene, when they save Stevie from the guinies who are trying to take their money. Its a sloppy fight, with punches barely landing, and people grabbing necks...hair...anything to get the upper hand. Just like real life. This movie does something else better than any other mob movie - shows the real ugly, nasty side of the mob. There's nothing pretty about the crime or lifestyle in this movie.
Sean Penn, though, is dirt terrible. Sorry, but you have to try to get someone who doesn't seem whiney and sissy-like. His voice and disposition just aren't right for this character (a 100% fictional character, by the way).