Anchor Bay UK really went to bat for the sphere-shaped set, printing full-page ads in trade publications and genre magazines. They also created a pretty fun flash game called Phantasm: Sentinel Hunt. While they have long since removed it from their website, yours truly managed to save the game before its removal and you can play it at the link below! The most unique part of the game? You play using your keyboard as a silver sphere hunting humans! If you manage to drill someone, you're treated to a brief clip from the films and then back to the hunt.
(requires flash, may not work on mobile devices)