When Kathryn and I were in Rome last winter, we used the public transit network extensively. The buses were most convenient for many of our trips, but we also used the Metro trains when we wanted to cover ground more quickly.
Kathryn and I recently spent seven days in Puerto Vallarta. Other than the rides between the airport and our resort, we did not use a taxi once. All our travel around Puerto Vallarta and the surrounding areas was on the local buses.