

Do you have a seat for her on the plane? If so, bring it on the plane and install it on the plane. If not, gate check it. We have checked ours with our luggage with decent luck but I really prefer not to do that since the last time we checked our stroller it was damaged. I think you would have better luck gate checking.
FWIW, the cost of renting a seat from a car rental company would be the same (or more) than buying something like the Cosco Scenera ($40) or Evenflo Chase ($60).
We do have a seat for her. But the Marathon is HUGE! Last time it took me forever to get it installed properlly and then the person in front of her couldn't recline because of her carseat.
We found this online....http://kidsflysafe.com/ so we are going to try it out on the way down.
I've decided to just gate check our carseat and call it a day.
Thanks everyone!
I also wouldn't rent ... here's a website that shows what you may be faced with at a car rental agency:
http://www.deliciousbaby.com/journal/2007/dec/22/advantage-rent-cars-frightening-car-seats/
I second the recommendation for buying a $40 Scenera w/5pt harness to travel with. If your dc is too big for rear facing, then another option that has taller top harness slots, is the Evenflo Chase ($60). Both are airline approved, fairly lightweight, and easier to travel w/than a big Britax seat.
Having said that, it IS possible to travel with a Britax and they do fit. The trick to installing them on planes is to get a seatbelt extender, OR at least make sure you flip the buckle over once before latching it so that it's easier to undo at the end. I also personally found that reclining the airline seat before installing the car seat and then putting it back upright after tightening the seatbelt as much as possible snugged it right up.
hth
Next time we'll ask for a belt extender!I am not sure if anyone posted this already, but the Graco Comfort Sport is supposed to be a great "travel only" seat. It weighs a mere 11 lbs and the base model is on sale right now at albeebaby.com for $69. Baby Bargains recommends it being narrow enough to fit in most airlines idea of a comfortable seat! We will probably get one for travel as we won't want to lug our Roundabout or Marathon around. 5 lbs is alot when you are lugging it around airports!