I disagree and say yes. We recently flew AA which has the same J seat LHR-LAX-LHR, out in F and back in J. My wife had our baby and thought the J seat was better than the F seat with the baby.

A car seat cannot be used in the CX J seat, they have a special child seat that can be attached to the J seat, but the seat cannot be reclined when this is installed. Also due to the layout it's difficult to see / assist the child if they are in their own seat.