I took my then, 2yrs 4mos old son, on a lengthy flight to Hawaii this past September and we took his Britax Marathon on the plane. We used the GoGo Babyz "cart" that hooks onto it that has wheels and it made it a breeze to push him around the airports. Luckily my husband was with us, so he was able to help with lifting and installing the seat. Once on the plane, though, my son barely had any room between his legs and the seat in front of him. A couple people reclined their seats back and that made it worse. I do have to say he was definately more comfortable in his own seat when he slept, but during our 8hr flight, he only got a 2 1/2 hr nap. So, this Thursday, my son & I are taking a much shorter flight to VA (2hr) and I decided to try the CARES FAA approved flight harness that you use in conjunction with the airplane seat's lap belt. I found a link on EBay where I can rent one for $13.99 + SH vs purchasing one for $70-80. It is quite small and only weighs about a pound and has it's own drawstring carry bag. You can look it up on amazon to read reviews. So, if you are taking a longer flight, a car seat is more comfortable in flight, but is a pain to lug around. A shorter flight, I would recommend going without a seat. Good luck!