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You've probably heard of this. This is that simpleton's JRPG that Square made for US Americans because they were worried people here were too stupid to understand JRPGs. Not surprisingly, it didn't do very well, and is generally regarded as one of the worst FF games.
This guy has an interesting take on it: Happy Video Game Nerd: Final Fantasy Mystic Quest
TL;DR: HVGN enjoys it as a no-commitment, lite JRPG for informal de-stressing. And more importantly, he theorizes that its failure (along with FFII(US)'s success) may have been instrumental in bringing more (and more faithfully localized) JRPGs to the west in the SNES era.