It didn't work too well in Pittsburgh during the Railroad Riots. The sheriff read the riot act but the crowd didn't disperse. The troops who were supposed to back him up decided instead that they couldn't fight their neighbors and generally mingled with the crowds. The State had to ship in troops from Philadelphia without local loyalties, who began to clear the crowds when the shooting broke out. The Homestead strikers decided to fight rather than disperse as well, as did numerous groups in the Anthracite riots a few decades later.

Granted though, for every movement that had to be broken up by force, there are probably many instances one doesn't hear about because reading the act worked.

(edit: here's the published report on the Pittsburgh incident. It might be more an atypical example of the usage of the riot act)