Most of the characters do, I think, fill their assigned roles very well. Tidus is extremely annoying, but he's supposed to be. Vaan is a bit of a blank slate, but he's supposed to be. Ashe is arrogant, vengeful and indulges her self-centredness under the guise of "helping her people" - but she's supposed to be like that. As characters, I like them, although I may not like them as people.
I agree with this. Some of the characters that get the most hate or polarised response are not bad characters as such, and I think it's because people do not distinguish between a well-written character and an admirable character. To me, a character is a tool for the writer to present the story; different characters can give different perspectives to the reader (or in this case gamer) on the same event. The above examples of Tidus, Vaan and Ashe, none of whom I like for their personalities, are still well-written characters. Take for instance Vaan, whom I used to dislike for being a 'blank slate', as LicoriceAllsorts says: I could fault Vaan for not being necessary for the main events of the FFXII storyline, but he gives us a unique
perspective that we wouldn't have if, say, Ashe were the main character. Vaan lets us see the world and events from a “commoner's” point of view, and his very blankness allows us therefore to follow the plot and setting as ourselves and project our own values (presumably also as "commoners") through him, like in an MMORPG (and isn't FFXII essentially modelled as an offline MMORPG?). I think Vaan is unique as a lead character in a Final Fantasy game in this respect, and I admire the writers for trying out this technique that breaks away from the tradition in the series of having strong characters whom we merely
observe.
Having said that, I tend to prefer a roster of interesting characters to observe
and relate to than a bunch of blank slates. Even well-written "evil" characters like Sephiroth or Edea are still relatable: we can admire their strength and mystery on a basic level, but understanding their motives and weaknesses is more important. Although I do not see myself in Sephiroth, I can emotionally relate to his reactions to discovering the truth about his origins. Knowing what he knows and his background, I can understand the world through his eyes. Tidus, although thoroughly annoying as a personality at times, is hardly different. I can understand his motives because he is well developed
and he gives a unique perspective to the player (actually he's not so unlike Vaan in this respect). I still don't really like him for his character too much, but that's not bad writing.
The characters I dislike are the ones whom I just cannot fathom. If the writers make no effort to explain
why a character is a way she/he/it is
in the game, then I think that's bad writing. We should not have to rely on in-game glossaries, the FF Wiki and Ultimanias to understand, except for minor details and extra points of intrigue. My main problem with some recent FF games is how they expect the player to leave the game itself to understand what is happening; instead of
showing and immersing us into their fantastic worlds, they too often merely
describe important events and expect us to follow along.
all we care is how strong our character are 
