It depends on which Roman law you are referring to.Here is a link that might interest you http://www.questia.com/library/book/ancient-rome-and-modern-america-a-comparative-study-of-morals-and-manners-by-guglielmo-ferrero.jsp

The Romans did not believe in long prison terms, as they proved too costly to the state and were seen as ineffective. Instead, they used criminals in the arenas and for labor, or executed them. They chose to exile people they didn't want in their society by sending them out of Rome. How beneficial was this to their society overall?

Honestly, I doubt this was much different from other ancient civilizations.  Or civilizations up to the early modern period.  We marvel at the kinds of torture and punishment used by people of ages past, and rightfully so.  But at the very least, we can say that they likely did their job of deterrence fairly well.  I heard that in the Middle Ages (perhaps in antiquity as well) there was a low chance of actually catching criminals, and so when they did, the punishments had to be harsh.  Today, we have punishments which fit the crime, but perhaps we have more crime overall.