A.D.
We used the CARES harness when my son was about 19 months old. He's average height(he was 32" tall at his 18 month check-up). He didn't like the harness at all, although distracting him with a granola bar helped for a few minutes. But we ended up just holding him for most of take-off; he fell asleep before we landed so we just held him then too. The problem was the buckles for adjusting the shoulder straps hit right at his neck, so I don't think it was comfortable for him.
I imagine it'll be better as he gets taller, and we're still planning on trying it for our upcoming trip, when he'll be 21 months.