I used the Bag Drop Shortcut at IAH a few weeks ago. You needed to be already checked in and could only get your bag tag there. The attendant was turning people away who were not already checked in or who needed anything beyond checking a bag. I'm not sure how it is faster than the normal bag-drop inside the terminal, but I guess it wouldn't get bogged down with people needing other assistance. There was only one attendant, so if there was a line you still had to wait.