Superman was never previewed because the producers didn't trust Warners with the film.

In an interview in 2000 Gibson described his impressions of Donner:

"Uncle Dick. The Omen (1976)
His break-through film was in 1976 with The Omen produced in the 'horror boom' following The Exorcist. However the original ending for the first film where Superman flew from accident to disaster was deemed to be missing a certain something by the film's independent financiers Ilya Salkind and his father Alexander Salkind.

Donner later continued reinvigorating the buddy film genre with Lethal Weapon (1987) and its sequels. "
In 2010 Donner's authorized biography You're the Director.