Op: I'm on the other side of the coin. In the last couple years, my 3.5yo daughter has flown ~100K miles in domestic economy, domestic F, int'l economy, and int'l business using the CARES harness. I like it, and so does she, but we live in a city and don't drive much, so she's not used to car seats. It's lighter than a car seat and safer than a lap belt (let's be honest; the real safety benefit is just during bumps, t/o, and landing). If you don't like lugging a carseat around, I say go for the harness.

As for the other criticisms, I suppose it's an opinion thing. I'm a little puzzled about the negative example above in which the poster mentioned using a carseat until 13 years old...