All shuttles have a bad feature: you wait until they fill up before departing--sometimes as much as half an hour; I have a recollection of using Showtime to get to Paris, I had to wait nearly half an hour for them to arrive and then another half hour to fill up. [Harrahs had a check-in area at the airport for most of their lv properties and they let me charge Showtime to room.]
Executive seems to be reasonably efficient--though they are not "affiated" with Harrahs (MGM uses them.)