I booked the service right when I boarded the flight, about an hour and a quarter before landing—an option that none of the other apps I checked offered, as they only allowed booking for the next day. On top of that, the price was 50% lower than what competitors offered. When I arrived at the airport in Saigon, a representative was waiting with the appropriate sign and escorted me to the fast-track lane within a minute. I have no complaints against the company about the fast-track line being long and crowded (across three rows of people who requested the service), but that is already the fault of the airport service providers.
I recommend the app.