One of my favorite games of all time. This Final Fantasy is definitely the best of all 3D FF's, and might even be better than my other favorite, II (or IV in Japan). The storyline definitely grabbed me the most, and the characters were memorable, unique, and all interesting.
The Materia system used in this game is genius, allowing crazy combinations (by linking them) and adding a tremendous amount of replay value if you want to get them all. The side quests, such as Chocobo Racing, are not only fun but they serve an important purpose as well. Getting the Gold Chocobo and being able to reach the "Knights of the Round" Materia is the most obvious reward.
The story featured many turns and twists, as well as relationships that mattered. The love triangle that existed between Cloud/Tifa/Aeris was nice, and the enmity between Cloud and Sephiroth felt tangible. And who in the world can forget that moment when Sephiroth drops from above and ends a certain somebody's life (for the 2 of you who've never played this game, I don't want to spoil it for you)? To me, that was one of the biggest moments in gaming.
Music was top-notch as well, and the "Aeris Theme" still rings in my head sometimes. Some people complained that the game was far too easy once you got the Knights of the Round materia, but they forget that getting it was completely optional. Plus, you had to devote hours to racing and breeding the Gold Chocobo, so to me it's justified. If Sephiroth is too easy, the Weapons will provide more than enough challenge. Another genius addition to the game, making bosses that are much more difficult than the main enemy. Totally optional, but you will definitely want to defeat them.
You can't go wrong with this game. Cloud, to me, is still the best Final Fantasy hero ever...and I think most people agree. Sephiroth is the most hated villain of all time IMHO, and by far the coolest. You are not a true RPGer or FF fan until you've played this one through.