We went through this same decision making process for our trip in November. In the end, I was glad we rented a car. We stayed at the Poly, and it worked great for our Magic Kingdom visits. Our second day we went to MGM, and decided to try the bus system. That morning we waited 30 minutes for an MGM bus. IMO, this is too long to ask people to wait. We may have just missed the prior bus, but I didn't see one on our walk to the bus stop. I didn't have any maps with me, and did not feel comfortable just hopping in the car without looking at a map. That evening, we returned to MGM for Fantasmic and drove - it was 10 minutes parking lot to door, and another 2 minute tram ride to the front gate. We used the rental after that for non-MK park days with the exception of Epcot (the Poly is a 5 minute walk to the TTA and the Epcot Monorail).
We, for one, were very glad we rented a car. It also takes the hassle our of scheduling your trip back to the airport with a limo or driver - you leave when you're ready. The cost is only marginally more money than a limo/driver: make sure you check the transportation board and Mousesavers.com for rental discount codes.
Hope this helps, Tx5