This system has been used in Minneapolis for a while already, and you can view the maps of local gunshots detected by the system.[1] Once in a while, the police have to test the system with police gunshots to calibrate the microphone network.[2] ShotSpotter appears, based on the experience of Minneapolis, to have rather limited usefulness for deterring or prosecuting gunshot crimes. But maybe it aids prevention of crime if police respond to where the gunshots are usually fired with preventive patrols.
[1] http://www.minneapolismn.gov/police/statistics/crime-statist...