You should search the archives for extensive discussion of the CARES harness, but the summary is:
1) Most kids who could be using CARES will need a carseat at their destination, so you might as well just
use that seat on the plane.
2) There is some concern that the CARES harness may not fit all children well and is extremely easy to misues. In fact, the photos on the company's own website show many misuses and inappropriate fits.
3) It can be a nice option for someone who is traveling with an older child who is 40-ish lb and who has a carseat waiting for them when they land (perhaps they're visiting family who have a spare seat in the vehicle for the child already).