We took our carseat in Jan for our 2 1/2 yr old son. When we got on the plane, the flight attendant informed us that our seat was not FAA approved. I specifically bought it for flights cause it was so light. Smart girl here didn't read the label that was clearly placed on the outside of the carseat. Turns out the type of base on the seat posed a problem.
So we put him in his seat, gate checked the carseat and I was super stressed out for about 10 minutes. Once I got his attention off the buckle, which is really easy for a little one to undo, he was fine. The good part was, he couldn't reach the seat in front of him, so he couldn't kick their seat and irratate the passenger in front. (I almost always end up with a kid behind me kicking my seat). The tray came to a perfect spot for him so he played with his Cars and he watched his DVD's the whole trip.
I didn't let him get out of his seat, except to use the bathroom once. I didn't want him to think it was ok to walk up & down the aisles otherwise I never would have been able to keep him in his seat.
Good luck with what ever you decide.