In the late 1950s and early 60s, 1930s gangsters were the rage. In addition to the TV show "The Untouchables", there were a lot of films that practically made heroes out of Dillinger, Ma Barker and other unsavory jerks of the era. One thing these depictions all had in common...they bore little semblance to the actual criminals! Why? Because their real life exploits weren't all that exciting...so they spiced it up by fictionalizing their lives--much like films of the 1930s and 40s did with cowboys like Billy the Kid and Jesse James. So, if you are looking for a history lesson or documentary about Baby Face Nelson, you should not bother with this movie.
If you don't mind that it's almost complete fiction, the film is modestly entertaining. Mickey Rooney's portrayal of the man is much like Jimmy Cagney's portrayal of Cody Jarrett in "White Heat"...completely over the top and more a caricature than realistic portrayal of anyone. This isn't so much a complaint...this is entertaining but it's hard to imagine anyone this antisocial and nasty being a successful gangster or working with any gang. This version of Nelson shoots people right and left, hates EVERYONE and is just plain nuts. The only problem with all this is that there is no room for anyone else in the film...even Dillinger! These other characters are one-dimensional and uninteresting. Overall, a dopey but enjoyable film. It has many faults but still makes for fun viewing if you have very, very modest expectations.