Convenience: I received a call from the driver one morning (10 minutes prior to scheduled pickup) saying he was on his way and passing a coffee shop and did I want anything...seriously? Yeah.
Oh, and they quickly take my information over the phone to reserve transport They know every flight number I've given them immediately from memory. They have a toll-free number in case my cellular phone battery runs out upon arriving back in San fransisco And they are always on time or early. And after three trips in three months, they remember me by first name on the phone.