To each his own....personally we rely almost exclusively on Disney transportation to the parks and back....however we love having a car for the resort to resort stuff....others may not agree, but especially when we travel during hot months I cannot stand that feeling of getting in a car that feels like a 350 degree oven,,,so the buses are great...but they are NOT great if you want to go to another resort for dinner, recreation, etc (it is certainly possible, just not quick or convienent)
If you are going to the parks in the afternoons, buses will not be crowded getting you there....coming home after Illuminations or Fantasmic is another story...buses will be packed most likely and you may have to wait for a little while to get on one.
However if you drive, you will have to deal with massive traffic exodus also....so I guess the question is do you prefer to wait in your own car in traffic and navigate through it yourself....or wait for a bus and then ride while someone else navigates the traffic?? (again, we prefer buses)