We have done it both ways, we have checked car seats and we have used them. If the car seat is FAA approved, you can use it on the plane. We have done this numerous times. This is the safest way for kids to travel, but I have to admit that it is a major pain to carry the car seat through the airport. It is also a pain getting on and off the plane with the seat. Some airlines will let you install the car seat before anyone boards, but there have been other times that we weren't offered this. During the first trip with our twins when they were two, we had them use their car seats on the trip to Orlando, but it was such a pain trying to carry two car seats and all the stuff that we had to carry, so we checked them on the way back. We didn't use a fancy bag to check their car seats, we were able to just check them as is. We don't have super expensive, fancy car seats though so we weren't worried about them getting dirty or stolen.