Don't read the book first

Rating: Null/10 (30 December 2002)

Major Spoilers below
If you want to enjoy this movie, don't read the book on which it was based before watching. The book is a true story written by Joe Pistone - the real Donnie Brasco. The film purports to be based on it but the story is so mangled that it hardly bears any resemblence to Pistone's truth.
At no point in the book does Brasco witness a hit or be asked to cut up bodies. Whilst he's given the contract to whack Bruno in the book, as in the film, he never actually finds Bruno in reality so is not put in the position mentioned in the film. If you read the book, you'll realize that these are vitally important points as Pistone would have been in serious trouble with the FBI if he'd taken part in a hit. Also, Pacino's character Lefty was not killed in real life (as is implied in the film) but died years later from natural causes.
Also, Brasco's motivation is totally different. In the film, he tries to get Lefty to leave the mob but in the book he specifically says, several times, that he would never do this as it would be futile. In the film he feels guilty that Lefty will be killed by his actions but in the book he knows that Lefty would whack him if need be so feels no guilt. Also, generally, Brasco's character is much more violent in the film. Besides all this, there are also several major plot differences which just didn't happen in reality.
I can accept a certain amount of artistic licence when transferring from book to film as they are different forms of media. But, as I said above, this film bears little resemblence to the reality of the book and suffers greatly because of it. (unless, of course, Pistone left a lot of truth out of the book but, I guess, only he knows that!)