I always get a rental car. The reason is that "dining" is a big part of our vacation. We usually don't dine in the parks but dine at other resorts. While the WDW bus system is great for getting to a park, it is often difficult to get to other resorts. Well, I should say it is difficult for me. When I go back to my room, change and get ready for dinner, I want to go directly to my restaurant. I don't want to catch a bus to the TTC and then find the next bus to the corresponding hotel. I have also heard horror stories of people trying to get to Downtown Disney during dinner hours--long rides, lots of people. Frankly, I'm just too tired to deal with that after a day in the parks.
I got a rental this year for $94.00 per week with taxes for the whole week. Car fits 5 people. In previous years, it's been a lot cheaper than that. If the cost was really high and I was on a tight budget, I wouldn't do it. But for that amount of money, it is worth it too me. Then I end up using it for everything. I drive to the TTC and get the bus to my park and it is very quick. Also is great for getting to the waterparks.