I know it is a wonderful convenience for anyone who is traveling for any period of time to arrange for the airport shuttle to pick you up and take you straight to the door at the airport.
However, I found them to be very unreliable with their services. Unfortunatley I was warned by several co workers about this service but I felt they were probably getting bad deal.
I arranged for the service to pick me up at my home at 4:30 for a 6:30 flight. They called and said the could not find my house, I gave them directions and found out they were completely on the other side of town. The did not seem to care about the fact that I needed to get to the airport they just kept calling with one excuse after another. After 45 minutes I told them to forget it I would just drive over. They honestly said "Well that's not very fair" I was running late getting to the airport because of their incompetance and they were telling me I was being unfair.
They ended up calling me a cab who took me to the airport but it still cost me $16 for one way.
So my advice would be if you are going to use the shuttle have back up arrangements made ahead of time in case they don't show up.