Flyjet
Technically,HUD's are great.I love the idea of being able to fly looking outside like I used to when I started flying .
What I know is that it's very useful during lvp ops on take off,we can go with 75 m RVR,on landing one can monitor the autoland and still watch for the cat 3b clues at the same time.Another plus is you can fly some really accurate visual apps with it.
Air Inter was a pioneer in its use (They had all their aircraft,starting with the Caravelle,the Mercure,the 330 HUD equipped) and they could go to cat 3 manual with it.
Air France,being less affected by the domestic network than AI decided not to equip the whole 320 fleet with HUD's,but the Orly based 320's kept theirs and every AF 320 captain is HUD-qualified.
That should give you some idea on the economics involved.
Cheers