We have one and have already used it many times!(seven or eight flights)
I think it is nice added security if you are going to carry your baby on your lap! You never know when unexpected turbluence will occur. It's also nice to have if your baby is sleeping, and you fall asleep, you dont have to worry about him/her rolling or falling off from your lap.
My son seems to be a sleeper on the plane, BUT he is starting to get more active, the last time we used it was during the holidays. I'm not sure how much longer he will be able to use something like that. He's pretty active now