I just got back from a plane trip today. Are you only flying with one? If so, here are some of the things I have done:
Choice 1: I have strapped my existing carseat to a portable luggage cart (the kind that fold) and used this as a makeshift stroller. If properly positioned on the cart, you can even strap your kid into the carseat. I've done this when I had only one child, it works great!
Choice 2: With some carseats and stroller (I've done it with my Marathon and Peg Perego P3), you can bugee cord the carseat to the stroller. Then, strap child into the seat and your carseat will roll through the airport.
Choice 3: (What I just did today). I used a PacBack (I think this is the right name), which you strap onto your carseat so that you can wear it as a backpack and then I just pushed our normal double stroller.
I've thought about purchased the sit-n-stroll, but they are pricey. From what I have heard, they are nice for the airport, but they are not great as either a stroller or a carseat. Plus, I find it difficult to reinstall my carseat for each car trip. I'd use this option only if I didn't really need a "real" stroller for the trip. It would be excellent for the airport, but not really good at any other time.
Let me know if you have any questions. We literally got home today, so everything is fresh in my mind!
Good luck,
Jennifer