I love FFIX. It's probably my secondmost favorite of the series (behind FFVI). I always thought of it as the "underdog" of the Playstation-era FF games. FFVII was just such a big deal when it happened (and was the introduction to the series for a whole generation of people), and everyone seemed to still be riding high on that when VIII came out (which I still think is one of the weakest FF games), that by the time IX came out, and had gone back to a more traditional feel with more cartoony style characters and less "edge" or "angst" it didn't grab a lot of people the way the previous two games had.
I thought, after Cloud and Squall and their "angry at the world" characterizations, Zidane was such a welcome change. And of course, Vivi... who's better than Vivi?
(No one is.)