I had a cheap one when my son was little, but only if I needed it on the other side. It was MUCH easier traveling with that instead of his bulky Britax.
If we didn't need the carseat on the other end, I, too, used a CARES harness. Borrowed from a friend the first time but liked it so much I bought one for us!! The think I like about the CARES (or just the seatbelt) vs a carseat, is that the tray table can't be used with a carseat (at least it couldn't with my son) AND it was much easier for him to kick the back of the seat in front of him in the carseat (and since he couldn't have anything on the tray, guess how he entertained himself?!?!).