I also enjoyed it. The character development is rather minimal, but I've read that this is not a bad thing, because very little is known about John Dillinger's early life. The compelling part of the movie is how the plot unfolds.
I've also read that Mann pays very close attention to gunfire audio. I guess that's not surprising since he did direct "Heat".
I thought it looked excellent, though some people complain about the "video" quality. Others would argue that perhaps it was done on purpose, to better transport the viewer to that time.
People were dressed very well. I want some of those suits.
The one gangster movie I am still waiting on to come out on BD is Miller's Crossing.