I don't really think FFIX is quite popular enough to be overrated. It has a small but vocal fanbase who love it fiercely, but I wouldn't say enough people herald it as the greatest thing to ever happen to Final Fantasy (it is, by the way), for it to be considered, by any stretch, overrated.

That said, I'll grant that the battle system can be slow (although the I'm such a fan of the mechanics behind the character growth and the static job system that I find this easily forgivable). I'll also admit that the dialogue can be campy at times, but I find that's a large portion of Final Fantasy IX's charm.

I'm going to flat out disagree about any nay-saying about the cast; as far as I see, Final Fantasy IX has the most fully realized casts in the series and is one of the few games where I feel that there's significant character development for a vast majority of the primary cast and a large portion of the supporting cast (Amarant feels at times unfortunately neglected and as the comic relief, Quina doesn't get the time s/he deserves for any real development). It's also a love story done right. The relationship between Zidane and Garnet progresses naturally with wonderful pacing and development (something I can't quite say about many other romance narratives in the series, especially FFVIII).

Final Fantasy IX is whimsical and humorous. I wouldn't go so far as to say its exactly a work of great fiction, but beneath its lighthearted exterior there's a level of maturity that's pervasive throughout. That kind of thing is hard to achieve and deserves appreciation.

On a final note, I do not understand how anyone could possibly regard Kuja as even remotely forgettable. He is one of the few (the only) villain in the series whose plight deserves sympathy and receives the same level of attention and detail as the rest of the cast. Most titles spend little to no effort giving their villains motivations (Kefka is just crazy--a charm in its own right--, Ultimecia is pissed offed for god knows what reason, and a number other villains such as Exdeath and Chaos just are) or perhaps piss poor one (I'm looking at you, Mama's boy!), Kuja just doesn't want to die. The emotions that drive him are human, so human that even the hero takes pity on him in the end. Kuja is easily the best villain the series will ever see.