Intelligent Tutoring Systems are effective. See this graph, drawn from VanLehn (2011)

VanLehn (2011) interprets these results as:

Human Tutoring (0.79 sigma) = Substep-Based Tutoring = Step-Based Tutoring (0.76 sigma)
Human/Substep/Step-Based Tutoring > Answer-Based Tutoring
Answer-Based Tutoring (0.31 sigma) > No Tutoring