I try not to be the guy crying "This game sucks! It shouldn't be canon!" I only bring this up because Roberta Williams left Sierra in frustration after she made this game, so she might not consider it canon, and if anyone could say "This didn't happen" and have it stick on an official level, it's her.
I tried playing the game when it came out, and I never made it through. It's not a King's Quest game at all, but rather a First Person Shooter, and, IMO, not a good one.
I'm playing through the games again in preparation for TTG's new entry, and I'm curious to know if I should force myself through Mask of Eternity, just to get the full story.