How useful are the AOA Indexers and indicators for jet pilots and civillians?
They’re super useful once you get used to them. No matter your weight, if you fly at the right AOA you’ll have the right stall margin. It is easier to see the indexer than the ASI, for control on approach. It acts like a pitch rate indicator too, if you are rough with the nose, so it helps to smooth out the flying. Responds much faster than the ASI. If you are maneuvering aggressively, it will give you that stall margin too.

Update with nice paper from AIAA Journal of Aircraft:

"Low-Cost Accurate Angle-of-Attack System", Borja Martos and David F. Rogers, Journal of Aircraft 2018 55:2, 660-665 


  ...The angle
  of attack for maximum lift-to-drag ratio corresponds to maximum range
  and maximum glide ratio for a piston engine- propeller aircraft.
  Hence, in a fuel critical situation and/or an engine failure
  situation, it is critical that the pilot have accurate access to this
  angle of attack.
  
  With today’s fuel costs, the Carson cruise [1] angle of attack, which
  represents the most efficient way to fly fast with the least increase
  in fuel consumption, is of significant interest.
  
  The angle of attack for maximum lift-to-drag ratio does not depend on
  density altitude, weight, or load factor but only on aircraft design
  parameters and aircraft configuration. Similarly, the angles of attack
  for minimum power required and Carson cruise depend only on aircraft
  design parameters and configuration...
  
  Therefore, a low-cost, accurate, full envelope, easy to flight
  calibrate angle-of-attack system is a transformative technology for
  light general aviation aircraft.