Our daughter was just a few weeks shy of turning 4 on our trip last September and she sat in her car seat on the plane. I am not a huge fan of flying so, I was happy to know she was safe and sound in her seat. She was very comfy and only asked once to get out to use the restroom. She either colored, watched the portable DVD player or I read to her. On the flight home she slept almost the whole way.
In the airport I put it in her stroller until we got to the gate. She just walked with us. We also use ME but, we feel better using her car seat, even if its only on the plane.
I am actually stressed she is over the weight limit for her car seat now. We are leaving in 9 days for WDW

and she is going to be sitting in the plane seat without a car seat for the first time. She is 5 and she is excited about it.
I wish her Britax Regent didn't weigh as much as my 11 year old.