We only live a few hours away, so we drive our own car (although we rented an SUV for one trip). We've tried Disney transportation a few times, but driving is so easy and painless, we drive everywhere.
We can be entering the parking lot of any theme park before we'd be boarding a bus back at our resort, and driving is WAY more comfortable than riding a bus...especially at the end of a long day.
We've taken the bus to MK, but frankly we think the hassle and waiting is more trouble than it's worth. Plus we enjoy taking the ferry or monorail.
The other big plus for us is being able to
easily come back to the resort during the middle of the day for pool/nap time. One round-trip by bus per day is tolerable; two round-trips would not be -- WAY too much wasted time. I think bussing makes more sense for Theme Park Commandos doing a LONG day in one park than it does for us.
Also, now that we will not be using
DDP, the car opens up a ton of dining options we wouldn't have otherwise.
I think you could probably pay for a rental car with your food savings from eating offsite. I haven't done the math, but I wouldn't be surprised if that was a net savings.