My comments are for domestic only (I haven't flown international with my daughter). For a Britax Marathon, ask for the seat belt extender from the get go. It will make it much easier to tighten up the belt to anchor the car seat. Otherwise as other posters have already noted, the seat belt buckle will wind up at an awkward point on the car seat and it will be virtually impossible to tighten the belt. My most recent experience was on a Delta 757 and the seat belt extender was a necessity.

Another trick we use is my wife and daughter temporarily stand in another row while I get the car seat fastened in properly. As long as your polite to other folks (even if you're temporarily in their seats), we have rarely had a problem borrowing their seats for five minutes while I finished setting up the car seat. Another option is to have your family wait in the galley while you mount the car seat. In any case, it is much easier and faster to setup the car seat without a two year old standing and squirming next to you while you work. YMMV....


--Jon

P.S. I have been using a Pack Bac by Cheeky Monkey for carrying the car seat on and off the plane and in the airport. If you're use to back packs (or day packs) it works great. The only caution is to be careful when you turn, etc. You will be a much wider load than your use to.