Do the NEO and MAX live up to their promises?

The Airbus A320neo and the Boeing 737MAX families have substantial backlogs, as they promise better fuel efficiency than earlier models.  New engine technology with the CFM Leap and Pratt & Whitney GTF promises 15-16% better fuel efficiency than the older CFM-56 and V2500 engines they replaced.  But do they live up to their promises?

The concept of a re-engined is attractive on many fronts.  The airframe essentially remains the same and familiar to existing mechanics.  Pilots can complete transition training within just a few hours rather than weeks, and the aircraft is compatible with existing gates and ground infrastructure.  A simple one-for-one replacement is very attractive for an airline.