I always drive to Disney from NY, so I always have a car. I have gone entire trips (13 or 14 nights) without ever using it, relying solely on Disney transportation. For the most part, the Disney transportation is pretty good, though there are occasions where you may wait for a very long time to get where you are going.
There have been trips where we primarily used Disney Transport, but also used the car some. Especially if we had early breakfast at a resort, or if we were going to Downtown Disney and the boats weren't running (we were staying at Port Orleans Riverside). Also, to get to off property locations like Wal Mart, and Old Town.
Last summer, we stayed offsite, and therefore, drove to the parks every day. As others have said, the only park that isn't terribly convenient for driving to is MK. Last year, we had an early breakfast at the Grand Floridian, and another at the Contemporary on two of our MK days, and so Valet parked at the resorts, and used boats / monorail to get to and from the park.
As long as you get to the parks early, before the mad rush of cars, driving isn't any less convenient that using the buses.