Christi,
If it were me in your situation, we would probably use the buses to get to the theme parks, but would use a car to go between most resorts, or between a theme park and Downtown Disney.
I would use a car in these situations because it would likely save us time. Many of the times you travel between resorts on the buses, you will need to use a theme park or Downtown Disney as a transfer point. Also, when you go between Downtown Disney and a theme park on the buses you typically need to use a resort as a transfer point (since parking is free at Downtown Disney, they don't want to make it too easy to get from Downtown Disney to a theme park [where you pay for parking if you aren't a resort guest]). Both of these transfers tend to make the overall commuting time a bit too long for my taste.
I will, however, share another perspective from a dad I met on the Monorail last weekend. He had driven with his family from Ontario, and hadn't touched his car since they arrived at WDW (8 days prior). His opinion was that if he is on vacation, he wants to leave the driving to someone else. Period. He and his family seemed to be very relaxed and enjoying their vacation thoroughly, and quite grateful that Disney has a very effective transportation system. I must admit that this dad had a fairly compelling point of view. ;-)
p.s. If you want info on helpful resources to figure out Disney point-to-point transportation options and times, click
here. You can also find answers to Disney transportation FAQs
here.