We were at WL 12/14 to 12/19 and we had no trouble with the buses at all. The buses are shared with Fort Wilderness or the GF depending on which park you are going to or leaving, but it did not cause a problem for us. We rarely waited more than 10 minutes for the bus.
One tip, if you stay late at the MK like we did for the 9:00 spectromagic, take the resort monorail to the TTC and then take the bus to WL, we waited for nearly a half an hour for the boat that evening. Other than that the transportation was great.
We rented a car and it stayed in the parking lot until the day we went to IOA.
Enjoy!