As someone who loves both VIII and IX, surprisingly I agree with Tres.
The thing is, IX's battles just take too long. I mean even if you use flee on every one you have to wait a long time for the battle to even start/load up before you can choose an action. And some of the small animations are really lengthy.
And don't get me started on the ridiculous trance system. 9 times out of 10 I triggered trance at the end of the battle but I was still waiting for the other 3 characters to use a command I had issued a period of two minutes ago that almost always killed the enemy I was fighting anyway. It might fit with the story/canon information given about "trance" but it's quite possibly the worst "limit break" system in any FF.
Other titles in the series just don't have any of those problems. In VII and VIII you can pretty much expect a command to issue an attack moments later. It sometimes gets so bad in IX that I forget which command I've actually issued. The other problem is that the encounter rate seems to increase from one PS title to another (I played all 3 one after the other this summer and streamed it, so check out my Twitch page if you want to see video evidence).
Here's a random YouTube video showing a battle. This is probably the average battle loading time, but sometimes the game decides to blackscreen on you for an extra 5 seconds:
Having said that, the mechanics/ ability system/ character specific skills related to battles are some of the best in the entire series, the story is amazing and the game is incredibly well paced.
It's well worth playing if you haven't before, even if it looks like you won't fancy it. It really is one of the better ones. Bear that in mind while voting. I should really be trying to "sell" IX to everyone since that's the one I probably want to play (that or X, having just done an VIII playthrough) but in the community spirit I'll play whatever wins in the polls!
EDIT: I should add, one of the really nice things about IX is the controls/exploration. It feels like they really nailed movement/talking/exploration with the controller. VII was incredibly clunky (there were no Analogue controllers for the PS when it was released), and while VIII used analogue controls it was really awkward at times. In IX it just feels really refined. It's something you have to experience to know what I mean, it's just all really smooth.
