We use the carseat. My 2 1/2 year old is very active, and it sends an unambiguous signal that he must sit along with everyone else on the plane.
Like the pp, I also have had problems with him kicking the seat ahead of him. I solve this by seating my DS5 directly in front of him. That way, it serves two functions: (1) Some poor stranger won't have him kicking the seat the whole time, and (2) It is sweet revenge for all of those times when DS5 kicked the back of my seat in the car over the course of the year. Bwa ha ha ha ha!!!!
Like the pp, I also have had problems with him kicking the seat ahead of him. I solve this by seating my DS5 directly in front of him. That way, it serves two functions: (1) Some poor stranger won't have him kicking the seat the whole time, and (2) It is sweet revenge for all of those times when DS5 kicked the back of my seat in the car over the course of the year. Bwa ha ha ha ha!!!!