We have always rented a car....you never know when a middle of the night run to the drugstore or supermarket will come up (DS is 5 and DD is 9)! We've thought about not renting a car, but, just the peace of mind of having the option sitting in the parking lot helps us sleep better.
That said, we, too, have found the buses to be a great way to get to the parks, especially from the POFQ. The only place we NEVER take the bus, is to DD. Whenever possible, and when time allows, we love to 'cruise' on the boats, but, failing that, we always drive....had only bad luck with the buses. They never seem to run frequently enough, and so much waiting to get back to POFQ.
Have fun!