Sean Penn play his top gangster role well, the only problem is that it's a stereotyped character, and quite boring. None of the cops are interesting except for Ryan Gosling's and that only marginally. If it wasn't for him and the beautiful Emma Stone, there wouldn't be anything interesting in this movie.
The action is well built, the soundtrack interesting, but that's it. The story is at best a support for shootouts and the fist fights. The dialogs are unimaginative.
You will be entertained, nothing more.