I suspect that the limousine companies quoted earlier in this thread are re-selling BFS VIP fast track with a little mark-up. There's no limousine company who has staff that are accredited and hold the required security passes to go airside and pick up passengers.

Even AoT Limousines (run by the company who operates the airport itself!) doesn't have staff with air-side passes who can go in and pick up passengers.

Thus I think money can be saved by separating the two things and cutting out the middle-man by booking the fast track directly with BFS and then booking a limousine elsewhere. In my case I go with the AoT limos as they gel nicely with BFS and I never had a problem.