Judging against just the other FF's and not video games as a whole, I'd give it a 7/10. Great things: The story is one of the most creative and most engaging FF stories, the writing is very good compared to other FF's, the graphics are wonderful, the sphere grid is addictive and delightful. Bad things: Spotty voice acting (Auron is exceptional and Tidus is horrible), not normal world map, wierd uncontrollable airship, no atb, the equipment is limiting and uncreative because each character has his/her exclusive equipment.
It has it's bad points, but the wonderful story makes up for them in my opinion. (Again compared to FF stories- it ain't exactly Tolkien but it beats the lame-o stories of some other FF's.)