Well, I'm not saying it's an exact ripoff, but there are many elements that are quite similar.
****CAUTION, SPOILERS FOR FF9/FFX****
1. Main character is from another place (zanarkand, terra)
2. Leading female is summoner (Yuna, Garnet)
3. Tough guy assigned to protect leading female has all the breaks (power break, armor break, mental break etc. Auron, Steiner)
4. Spheres/circles are used in some fashion (sphere grid, magical stones)
5. The characters are pretty much the same (skill-wise, there's nothing new)
a. Zidane = parts of rikku(stealing), but has the leading role of tidus
b. Vivi = Lulu. Do I need to get any further in-depth?
c. Steiner = Auron. Sworn guardian, uses a huge sword. No questions asked.
d. Garnet = Yuna. Leading female, summoner. Pretty obvious.
e. Quina = Kimahri. Uses blue magic, and comes from a weird race.
f. Freya = Wakka...but kind of not really. The "jump" command can be thought of as an aerial attack, but it really doesn't completely match up.
g. Eiko = parts of tidus (haste magig), but otherwise a useless character (actually, more useful than Garnet, but since she is the 2nd summoner to come on the scene she is more useless),
h. Amarant = Rikku..kind of. They both throw stuff, and that's sufficient.
So there's a few differences and a few similarities. But other than the minimal differences, the main plot remains the same.
FF9's plot outline: "A bunch of adventurers led by a thieving man and young summoning girl are trying to stop the world from being destroyed by forces from another planet."
FFX's plot outline: "A group of people guarding a summoner who is attempting a pilgrimage to an ancient city to prevent the further destruction of the world."
Yeah...pretty much no different.
