Which airline are you going to use?. I would ask them if you can actually use this harness on the aircraft in any event. You also need to see what their policy also is with regards to take off and landing.
Yes, I have used a Cares Harness before. Good piece of kit and we used it quite a bit when DS was aged from about 18 months upwards.
My only tip would be one of you gets on the flight before the other with the child so you can set the thing up without bothering people at the back of you as you will need to lean over into their space to ensure that the belt ins't twisted etc.
You should not be restricted on any airline using the harness.