As of now I'd give it a B+ but I do think that it will get better. The cast is excellent - everyone. Johnny Depp is charming and funny and Bale does a fine "serious man-on-a-mission".
It starts off with a dramatic breakout of Johnny's gang from prison with a pretty killer Mann caliber shootout, and then we meet Christion Bale as a rising star in the FBI tapped to lead the manhunt for Dillinger. J. Edgar Hoover (Billy Crudup - excellent) hand picks him for the job and while the scenes with Hoover are good and somewhat entertaining by themselves I don't think they necessarily contribute that much to the movie. There is obviously supposed to be some tension between Hoover and Bale's FBI guy over how Bale should operate and what kind of agent's he should use, but it's underdeveloped and when there's a final showdown between the two men in the end you just don't care much. To make that subplot worthwhile would probably just extend the movie, which is moving in the wrong direction, so cutting it back or out might be a better choice. Just a guess... I don't know anything.
The biggest weakness overall though was the adversarial relationship between Bale"s FBI guy "Melvin Purvis" and Depp's Dillinger, which is surprising because Mann usually does that so well - setting up two competent adversaries on a collision course. Here Purvis keep's somewhat blowing it, making the wrong choices and being countered and sometimes scolded by guys that are supposed to be working under him. I guess they're trying to show how desperate he's getting to catch Dillinger under pressure from Hoover, but it was at the expense of the tension between Purvis and Dillinger which was the direction they seemed to be going early on. There is a Deniro vs. Pacino moment early in the film, reminiscent of the diner scene from Heat, where the two lead actors meet and challenge each other - but after that their relationshp is pretty much dropped. There's no setup's where on barely outsmarts the other, no taunting or signals sent from one to the other. I would have liked to have seen more of that. Also, I just never really got any idea of what drives Bale's character... he's somewhat underdeveloped.
The best part is the love story between Dillinger and actress Marion Cotillard from La Vie en Rose, who plays a bright girl leading a fairly simple life who Dillinger falls in love with (rather quickly - but it's humorous and it works). Their relationship is well developed and the most effective storyline, interleaved well in to the movie as to not take away from the action for too long... which is important because that is what michael mann does best and there's plenty of it in this movie. Only one actio scene didn't work well for me, a nighttime assault by the FBI on a cabin hideout, which was confusing and dragged on for too long in the cabin location before turning in to a pursuit and gunfight. Mann usually sets up his action scenes so well that you know who's shooting at what and where people are located as the adversaries try to manuever for positions, but that wasn't the case here. I imagine it can be fixed in editing.