The interesting role for James Cagney is probably the main reason to watch "Great Guy". The role in itself is a fairly uncommon one for a leading role, and Cagney gives it his own distinctive style. The movie overall is a solid if rather predictable crime drama, with a couple of interesting details.
Cagney plays a new official in charge of the bureau of weights and measures, which is a relatively creative choice for a movie hero. As Cagney goes about investigating various instances of fraud, his character gradually takes on more and more of the tough guy persona that you associate with Cagney. At the same time, the stakes become ever higher in his battle with the sources of corruption.
The supporting cast is adequate, but they are generally overshadowed rather easily by Cagney. Mae Clarke is relatively appealing as Cagney's fiancée, but she mostly has to react to situations, since the script and dialogue don't give her much more to work with.
The movie as a whole largely follows a familiar pattern, and with a lesser star it would have been a rather routine affair. Cagney brings it up a couple of notches, and his own performance certainly won't disappoint anyone either.