I have not really made up my mind about it because it is an area of policy I have not studied sufficiently to develop a well-informed decision. As a first impression, however, I lean toward opposition on grounds of the criminal justice system being capable of screwing over morally excellent people who had isolated lapses in sound judgement versus relatively vicious folk who present a greater long-term threat to both themselves and others. People who show themselves to be such a threat need to be isolated from the rest of society - not as a punishment so much as a measure of last resort if they are not responding favorably to attempts at rehabilitation and reintegration. Mandatory sentencing seems to get in the way of bringing victims and offenders together to flexibly work out their grievances and settle on what needs to happen moving ahead.
You are absolutely correct and who better to teach you about successful crime than those who have been there before and spent time in and out of jail their whole lives. Aside from practicality, mandatory sentencing removes power from the judiciary system and puts it in the hands of the legislative system. It actually mandates a lack of judgment rather than letting judges use judgment.