Flight attendant checked that it was not expired, and we had to buckle in on window seat. It was not easy to buckle in properly and getting it out was a lot of fun, let's just say my wrist was very sore.
However, my ds sat through the flight (just under 3 hours) and we had no issues. If he was in big seat, things would have been entirely different.
So, if you have the choice, use the car seat. I would also recommend asking for bulkhead seating because they tend to kick their feet which might disturb the people in front of you. If you can't get bulkhead, just warn the people ahead of you. Better he is seated comfortably in familiar car seat , than forcing him to sit in big airplane seat. Turbulence can happen at any time, so better to be safe.
a portable dvd player with barney or teletubbies also helps!!