I've always lijked the idea of Simon's Quest but felt the game was just too limited by the technology. I've only ever played through it once.
I remember that even back then, I loved the idea of character customization and growth, and I loved being able to equip stuff in the game, rather than picking up random weapons, and non linear exploration. This was light years ahead of its time, but also kinda too frustrating to play through.
Because of the annoynig factors of the game, I was so relieved by what they did with III, which also featured some customizaiton with the sidekick options and branching paths.
But definitely, this game is in the DNA of a lot of Castlevanias that came after, especially from Rondo on.