I traveled with my 3 1/2 and 7 year olds this past week and did schlep the younger's car seat for on the plane use. Although ours is a bit heavier b/c of the adjustable base, I found that it made her ride much more comfortable and she was able to rest during the ride as well.

I either carried it over one shoulder, like a back pack, or I flipped it downward over the rollerboard and used the tether to help balance/secure it while we walked through the plane & airport.

Plus, I had the added benefit/comfort of having the seat for our rental car - wthout having to wonder if the car rental seat was a safe model/clean/etc.

(i also brought my son's booster seat (just the bottom) to insure he rode safely too, stowed it in the overhead so we had it for the rental)