Really depends on the attraction and timing.
For Smugglers Run, Web Slingers, Grizzly River Run, and Splash Mountain seems to consistently shorten the wait enormously.
For Radiator Springs, it seems to be great early in the day, pretty long in the evening.
For Goofy's Sky School, seems to be a crapshoot depending on the party sizes in the standby/Genie line.
For Incredicoaster and Matterhorn, seems like single rider takes as long or longer than standby.