I did find the evolution of the plot not very convincing as Cagney's character goes from extremely suspicious and brutal to a very trusting guy (at least towards his new cell mate - the police informer). Everything is explained, and the death of Cagney's mother might add vulnerability to his situation but it still felt a little bit conceived to me.
The actors themselves were extremely interesting to watch, I especially liked the mother. Another thing that caught my attention was the brutality of the main gangster and how it was dealt with on the screen. It got past censorship more or less uncut I guess, but the events had a very "modern violence" feel to them.
The scene with the hot steam in the beginning was very interesting as it led even more depth to the ruthlessness of the gangster, serving little else. Also the showdown with the explosions was quite impressive. To me, Cagney's character seemed like the prototype of a Bond movie baddie.