I think it depends where you are staying if you need a car or not - we went two years ago with three daughters, we stayed on I-Drive and used the I-trolley to go to SeaWorld, Universal, Wet n Wild and Islands of Adventure. We used taxis to the Disney parks which cost us 80 dollars return each time we went, therefore we didnt see as much of the Disney resorts as we would have liked (such as Boardwalk, Old Town, Downtown Disney etc).
We are going on Thursday this week, staying in a Disney Hotel and have hired a car because it works out cheaper than what we paid in taxis and bus passes for the two weeks (my youngest was free on the I-trolley) but it cost about 28 dollars each for four of us. I think all in all it cost us about 500 dollars in travel last time.
Good luck and hope you enjoy your holiday xx