I love having a rental car in WDW. We do so many things that we would not do if we used Disney Transportation. We visit all the resorts, we go to the supermarket for food in the room, we book restaurants at certain resorts we would not book otherwise.
We have gotten used to the freedom of having a rental car and going anywhere at anytime. I dont mind Disney Transportation if we are going back and forth to a park, but we do a lot of other things, and the car makes that very easy.
I love to drive around and find everything and get a feel for how big it is in WDW.