I'm really liking the Duelist prestige class in P:K. What's interesting is that it's a melee class that relies on Dexterity and Intelligence. You get bonuses to your armor class from both of those stats, and they stack. The justification for Dex-based AC is well known, your character is good at evading and dodging attacks. What's less common is using Int as well for that purpose. Basically, Duelists are tacticians, they can dodge attacks not only because they're agile, but also because they've studied melee tactics, or something along those lines. So, theoretically the armor class of a Duelist can be higher than that of a Rogue.
Duelists also get some interesting skills, like Parry. Essentially, you only use one weapon and no shield. But if you score an attack against an enemy, then on the next round, when the enemy attacks you, you can use your weapon to try to Parry their attack, taking no damage. Instead of using your armor class, you make an attack roll. If your roll is higher than what your opponent rolled, then you Parry the attack. And if you don't, then your opponent still has to surpass your ridiculously high armor class that's based on Dex + Int. (It's actually a little more complicated than this. If you can attack several times per turn, then you can choose to not use one of those attacks, and you can use it instead for Parry, but you get the idea). Long story short: Duelists are very difficult to hit.
Another interesting ability is Riposte. Basically, if your opponent attacks you (and if you successfully Parry the attack), then you can immediately attack them back, on their own turn, instead of waiting for your character's turn (it's technically an attack of opportunity).
The downsides of this class? Well, if you do get hit, then you better get some healing right away, or you're not going to survive if you get hit a couple of more times. Unlike a fighter or a barbarian, you don't have a ton of hit points. But this isn't unique to Duelists, Rogues also have this drawback.
Another drawback is that you don't get Sneak Attack, like Rogues do. But this is fine, you're basically replacing Sneak Attack with a bonus to AC based on INT + DEX, not just the latter. So, it seems balanced.
TL;DR: Duelists are sort of like defense-oriented Rogues. They do less damage but have higher armor class.
EDIT: Actually, if you multiclass, you can get Sneak Attack, but not from the Duelist class itself, you get it if you take some levels in Rogue (or some other class that also has that ability)