The audience never really connects with any of the actors. The movie does a reasonable job at telling the JD story. However, the viewer does not leave the theater feeling anything.
Far better examples from the same time period are "Road to Perdition" where we really care about Sullivan (father and son) and want them to win. Also "The Untouchables" where Malone and Ness bond like father and son. Both of these movies have real stories.
Public Enemies is just a chronicle of JD and in the end good triumphs over evil because a former ally is now in need. Hoover is a politician, Purvis is evil and disconnected and JD runs roughshod over the whole bunch until betrayed. Where is that spark that drove the writer to create the screenplay?