The Fly Safe harness is very easy to use. Since I was travelling with 2 kids (including a newborn who was seated on my lap) and renting car seats through the rental car company at our destination, the harness was ideal. Even though my toddler is big for his age, the seatbelt in the plane was too large for him, and so, I'm not sure it would have been entirely safe if used without the harness. I gave the harness a 4 out of 5 rating because my toddler kept sliding down on his seat, although, admittedly, this might have had more to do with the seatbelt being too big for him. That being said, it is worthwhile to note that my toddler had the harness on for the ENTIRE length of one of our 7-hour flights (except when we went to the WC); he slept for 3/4 of the flight.