We usually drive down, so we do have a car. We DO NOT use it to get to the parks, though. The busses are much easier, in our opinion. No parking hassles, traffice, etc. We really like using Disney transportation for getting to and from the parks. It's nice to let somebody else do the driving, especially at the end of a long day. We have never had to wait long for a bus or boat to or from a park.
We DO use our car for getting to Downtown Disney, other resorts for meals, the Boardwalk, the water parks, and the mini golf places. Although you can get all those places with Disney transportation, it is much quicker to use a car. We have done a few trips without a car, though, when we have flown down. It wasn't too bad at all. It just took a little extra planning and time to get to some of those other places. We probably won't ever bother to rent a car if we fly.
Be aware that if you drive your own car to the Magic Kingdom, you will have to park at the Ticket and Transportation Center and take the monorail or ferry over. The bus will let you out right at the park.