As a kid, maybe a 7. I played Final Fantasy first and it's the better game of the two, by a fair margin.
As an adult, hovering around a 5. I don't have time for grinding an NES RPG and if I did, this one is probably the most boring to do so on. It does get some decent nostalgia points, as well as points for audio; the sound effects are memorable, as well as the music.