For someone who criticizes SquareEnix's poster child series as much as I do, I sure do wind up defending its characters strangely often.
Cait Sith. If you had to list the most universally hated characters from the Final Fantasy series, he'd be up at the top with Final Fantasy 9's Quina Quen. Nobody can seem to stand him. And, as is often the case with overly disliked Final Fantasy characters, all this scorn is nearly entirely undeserved.
So, here's what I gather are the major reasons to hate Cait Sith:
1. He looks stupid.
2. He's not useful in combat.
3. He betrays the good guys at one point.
4. He's annoying.
So let's see. First off: looking stupid. I probably shouldn't even dignify this with a response--if you're a shallow enough person that how a character looks is going to be the major determining factor in your opinion about that character, then you're not worth wasting words on. Nonetheless, I would like to point out that, beyond the fact that it's moronic to judge a character's worth by how they look, this is FINAL FANTASY. Stupid-looking people are not exactly a rarity in these games. If you're going to dislike Cait Sith for not looking particularly awesome, you might as well hate the majority of the characters in the series, because most of them look ridiculous one way or another.
Next is my personal favorite, his use in combat. I actually find this to be an even less intelligent reason to dislike Cait Sith than his looks. I mean, to start with, I really don't see the point of liking or hating someone according to how immediately useful they are to me personally. But putting that aside, this opinion is wrong on multiple levels. To start with, Final Fantasy 7 is a game where differences in combat abilities between characters are so minimal that they almost don't exist--the only major skills that distinguish one character from the next are the Limit Breaks, which are only rarely available to use. Other than that, combat skills are determined by the Materia the player puts on a character. So for most regular battles, and the majority of each boss fight's duration, it's the abilities granted by Materia--most prominently, the Magic and the Summons--that determine how useful a character is. So Cait Sith's only going to be about as useless as the player is inept.
So in general, it's Materia, not Limit Breaks, that's going to determine how useful a character is going to be for the long haul. So, while the Strength stat is handy if you put on the right Ability Materia to enhance the regular attack (and the good ones for that only come late in the game), it's the Magic stat that more determines how powerful a character is going to be, since most offensive Materia provide magic spells and Summons. And hey, guess what? Cait Sith's Magic stat is one of the best in the game. So unless you're really fond of tripling how long each battle takes by just using the Attack command every time and ignoring Magic and Summon (and E. Skill, always damn handy), he's going to actually be one of the more useful characters you can get.
And about his Limit Breaks, which everyone says suck: for his Slots Limit Break, there's a tiny chance of getting a match-up known as Death Joker. Now, Death Joker is simple: it kills everything in battle. Enemies and your team both die, no questions asked, doesn't matter what enemy it is. This can be countered by the famous Materia combination of Final Attack + Phoenix (or Final Attack + Life, I guess, but that's not nearly as handy), which makes it so that when a character gets KO'd in combat, they cast a spell as they die--in this case, Phoenix, which restores them and any other party members at 0 HP to fighting condition.
So, basically, Cait Sith can pull off a Limit Break that will kill a Weapon, and can be countered by a common Materia combination trick. Yeah. That's not precisely what I would call useless.*
Lastly, his betrayal. Okay, yeah, Cait Sith betrays Cloud and company. THIS is actually a legitimate strike against him. Acknowledged. That said, however, it's still not fair to hate him for it. To begin with, the betrayal isn't his final defining act. He pulls his traitor act while you're still on the first disc of the game--every moment after that, he's working to help Cloud as best he can, making sure that those dear to Aeris and Barret are safe, becoming Cloud's informant about Shinra's activities, fighting alongside the group, and so on. He makes up for his misdeed plenty times over, no mistake about it.
It also makes no sense to hold it against him when other RPG traitors rarely have such grudges held against them. I mean, just going by the Final Fantasy series, look at Kain. Kain from Final Fantasy 4 turns traitor TWICE during his game, and while mind control is involved, it's only made possible because he already has within him the negative emotions of jealousy and anger that betrayal is created from. The way I see it, that makes it a more earnest case of betrayal than Cait Sith's--with Cait Sith, he probably doesn't have much choice, given that he's employed by a company with military power that is not particularly nice to anyone who doesn't play ball with it, and Cait Sith's actions and dialogue throughout the game paint the picture of a character who wants to help people and do everything in his power to keep others safe from danger. His betrayal isn't motivated by bitterness the way Kain's is. Yet Kain nearly never receives the kind of badmouthing over his multiple betrayals that Cait Sith does about his one.
While we're on the issue of comparisons, other RPG traitors often tend to do worse things when they turn on the good guys. I mean, think about it. Cait Sith's traitorous act is to feed Shinra information for a while, and then to give the evil corporation a key item that it needs to get at the Black Materia, which Sephiroth, who both the good guys and Shinra are opposing, wants to destroy the world with. End result? Shinra keeps up-to-date with Cloud's group of do-gooders, which it basically was doing fairly well anyway before Cait Sith came along (the Turks know he's coming in the cave outside the marshes, Hojo sights Cloud in Costa del Sol, and they obviously know he's going to the Gold Saucer since they place Cait Sith there to find him), and Shinra gets a head start on obtaining the Black Materia--which, while bad, would still be better than Sephiroth getting it. Compare that to the betrayal of, say, (MAJOR BATEN KAITOS 1 SPOILERS) Kalas from BK1. Kalas is a double-agent the entire first half of the game, and his betrayal of his friends, which is motivated solely by his own selfish desires, results in the releasing of a freakish, malevolent god's power, the capture by an enemy military of the girl that was digging Kalas up to that point, and the finger being given to you, the player, as Kalas actually attempts to forcibly eject you from the game's events. Yet you never hear of people holding a major grudge against Kalas for how royally he screwed everyone over (even considering the much smaller fanbase for the Baten Kaitos series). So why the ever-lasting hatred for Cait Sith on this count?
Finally, the annoying factor. Well, frankly, I'm not sure I get it. I mean, Cait Sith's role in the general plot and dialogue of the game is on the lower side of average, and his major acts and speeches, from what I can see, are focused on his betrayal and his desire to help people and make up for his earlier deception. Not all that annoying a focus, and his personality itself is a fairly mild-mannered, yet energetic, guy who adopts a southern accent at random. And without him, Cloud and company wouldn't have gotten the Black Materia, Barret wouldn't have escaped Junon during the Weapon attack, and Elmyra and Marlene wouldn't have been kept safely away from Meteor's ground zero. He also provides intel to Cloud about Shinra's movements once his connection to them is revealed. So it's not like he was just some hanger-on; he significantly contributed to the world-saving effort, more than several other members of the party who are rarely criticized. Honestly, he doesn't have a loud enough or dull enough personality to be all that annoying. I just think that most people who say he's annoying do so because they dislike him for one of the other criteria I've mentioned.
Cait Sith gets way, way more resentment sent his way than he deserves. That's what I think, and that's what I stand by. People forgive poorer characters for worse acts, and half of the complaints people have against him are groundless, stupid, or both.
* Yes, I know, the chances of Death Joker are crazily rare. But I'm willing to bet that if you compared the time it takes to keep trying for a Death Joker to hit the Weapons against the time it takes to get the levels and the rare Materia and the levels for said Materia, along with the best weapons and armor and such, plus a goodly amount of healing items, all of which is required for the standard way to take out a Weapon, you're probably going to spend less time waiting for the Death Joker. The time it takes to get a goddamn Golden Chocobo alone...