We've rented a car once and used the transportation twice, both times I said we should have rented the car. It always seemed that while I was walking up to the bus stop at our resort the bus I wanted was pulling away and another 20 mins was being wasted... The same was true for rides home from the parks, it always seemed there would be 6 busses to whatever resort was before or after our waiting area to our 1 bus. Also if I wanted to visit downtown disney from a park I had to actually take a bus to a resort and wait for the DTD bus there, that may have changed for some parks now, I believe animal kingdom has a DTD stop now.
Yes your on vacation and supposed to relax, but those days go quick and there is lots to see, I don't want to not see something cause I'm waiting on a bus.
This October I've already got my rental car reserved at the car care center and will be checking for lower prices weekly. I don't really want to take a taxi to the center but so far it is the cheapest route, last year (even though I didn't do it) it was cheaper to pickup at the airport and drop off at the swan n dolphin
If picking up at the airport I suppose I won't be able to use the baggage pickup though as I wouldn't be doing the magical express, it is so nice not having to stop by a luggage carousel