For the last four years, the School of Physics at Georgia Tech have been preparing new Graduate Teaching Assistants (GTAs) through a program that integrates pedagogy, physics content, and professional development strategies. Here we discuss various assessments we have used to evaluate the program, among them surveys, GTA self-reporting, and end-of-semester student evaluations. Our results indicate that GTAs who participate in the program find its practical activities useful, feel better prepared for teaching, make use of learner-centered teaching strategies, and receive higher scores in teaching evaluations.