This film stars a wonderful Johnny Depp who pretty much carries the film for most of its near 2 1/2 hours. It is visually stunning (with a few caveats, of which more later) often compelling, but there seems to be something missing. A lack of "back-story" is not the problem as Depp makes clear in his own mini auto-biography delivered to Billie "What more do you need to know?". It seems to be a problem with tempo and the development of drama. There are too many shoot-outs, too much of a concentration of the scenes dealing with character in the early part of the film, and too little organisation of the order of scenes, leaving the viewer with a film that lurches from one thing to another rather than allowing a dramatic line to unfold.
Christian Bale as Melvin Purvis plays a rather dull character, and Billy Crudup (who "Watchmen" fans might recognise despite his failure here to turn blue and expose his genitals) was more engaging and watchable, for me. But there is no real sense of a dynamic, dramatic conflict between Purvis and Dillinger. Marion Cotillard is quite good as Billie, without conveying much of what is supposed to make her relationship with Dillinger work. It is a film with lots of great scenes, which don't add up to a great whole. Most viewers will surely empathise with Dillinger (the aspect that he is more hated by much worse criminals than by ordinary citizens no doubt helps) but since towards the end he doesn't seem that bothered about what happens to him, it becomes harder for the viewer to care.
On the BD side, this film was shot on HD cameras (apparently the 35mm scenes were tests and did not make it into the film). This makes for outrageously good visuals at times. Winstead bending down to the pavement in close-up, or in the interrogation room with Billie are about as good as HD gets. Almost hyper-real in the sense of depth, plasticity and three-dimensionality. When Mann wants it so, the film showcases the format with natural colours, sharpness and brilliance of image. But at times there is quite heavy filtration, mainly yellow for darker, indoor scenes. And despite the much-heralded grain-free guarantee from going all digital, some of the night scenes have very aggressive digital noise which is more unsightly than most grainy night-time scenes I have recently seen. Audio quality, as another reviewer mentions, is a mixed bag. The soundtrack is 5.1 DTS HD Master Audio - DTS HD here appears to stand for Dialogue Towards Silence Hail-of-bullets Deafening. Every shotgun shell here would do duty for a nuclear explosion in many other films, while the dialogue is far too quiet. I was constantly adjusting volume, which was very annoying.
What remains is an entertaining but imperfect film which has a hollowness, the sense that something is missing and this prevents the film from being as great as Depp's performance deserves. As a BD the sound issue is annoying, but the picture quality is very good, bearing in mind the few caveats above. Worth a look, but don't put it top of your list.