Rialto, California is one of the few places where the use of police cameras has been studied systematically:
In the first year after the cameras were introduced here in February 2012, the number of complaints filed against officers fell by 88 percent compared with the previous 12 months. Use of force by officers fell by almost 60 percent over the same period.
[...]
During the yearlong study, half of the city’s patrol officers were randomly assigned to wear body cameras each week, and instructed to turn them on whenever they made contact with a civilian.
Officers used force 25 times, down from 61 over the previous 12 months. And those wearing cameras accounted for 8 of those incidents.
Slightly off topic, a post on the topic of CCTV cameras and how to avoid them.