We just got home Tuesday from a week stay at POFQ... we've always rented a car in the past but this time I thought we'd try using the transportation. The ME went smoothly and we were at our resort about an hour after our plane landed, but by the third day of using the buses I couldn't stand it anymore and we ended up getting a rental car.
Maybe we just had some bad luck with the transportation, or maybe I was just used to the convenience of having a car, but I would NEVER go without renting a car again. The buses were almost always crowded and we had to stand (not an easy thing to do with just me and my 4-year old and her stroller); we missed our PS at Chef Mickey's because it took almost an hour and a half to get there; we tried to use the ferry twice to DTD but both times we stood in line for about 20 mins. and then they stopped the ferry service because of thunderstorms; then one night leaving the MK we tried to get to the Poly for dinner but the monorail was having "technical difficulties" and we had to wait about 30 mins. until it started running again.
If you are planning on doing a lot of dining at the resorts, I would definitely keep the rental car - it's just my opinion and I know a lot of people feel otherwise, but it seems like we wasted several hours a day using the transportation; once we got the rental car, we could get almost anywhere on property within 10-15 mins. and the parking and trams were really not a problem.
You could always go ahead and try using the buses and if it's not for you, it's very easy to rent a vehicle from the MK Car Care Center, which is what we did.