In our many weeks at OKW, we have always used Disney transportation, even for the few trips we have had a vehicle. For resort-to-resort transportation, we always allow an extra 30-45 minutes for travel time, but find the buses more than adequate.
In addition, we've stayed in all areas of the resort and have no qualms about using buses.
If you have small children with lots of gear or have some in your group with mobility problems- a car would be useful (that is true for ANY resort at WDW, IMHO). If you will be traveling off site to other attractions in the area, a vehicle would be useful.
Disney transport does a great job.
Enjoy!