If I understand it correctly, if you have an ADR prior to a park opening, you have to take your reservation info with you to the gate and they let you in. I we have a character breakfast at Animal Kingdom at 8:15am on a day when the park opens at 9. When we booked it, the cast member on the phone told me to just take that to the AK front gate with me. It'll be nice to be in the park that early and can hopefully get in line fairly quickly to Everest...
We've never used the Disney Transportation because we are renting a car. It's just more convenient if you want to leave parks at your leisure, plus we're going off-site a lot and don't want to hassle with waiting for cabs or other transportation schedules. It's all about convenience for us.