I dislike called shots. For the most part, I think they are unnecessary and incompatible with Pathfinder's abstract combat system. Hit points and attacks per round are abstract methods of resolving combat, the whole system is abstract. If you want a very realistic and complex and non-abstract system of combat, play GURPS. I've spoken before about hit points and how they represent far more than just your physical toughness, that they incorporate luck, endurance, skill wearing armor and using it to absorb blows, etc. I've also spoken before about how attacks per round are another abstract concept that do not represent how many attacks you actually take in a six second round. Critical hits are another abstraction of combat in Pathfinder and DnD, they represent luck, getting in a blow at someone's head or face, a particularly well timed/aimed blow that slips through a chink in armor to hit vitals and so on. The critical hit modifying feats such as Blinding Critical, Stunning Critical, and Bleeding Critical represent hits to the eyes, a hard head blow, and getting a deep cut into meat or crushing/cutting an artery. Increased Critical vaguely represents increased skill at striking vitals. To integrate a called shot system into Pathfinder's combat, you have to cut out stuff like critical hits and everything associated with them. I'll condense my thoughts here: combat is abstract, called shots are not, saying a game needs called shots also implies that your character is actively making sub par attack choices during combat, that he is actively making an effort to strike weak blows, rattle his sword against armor too thick to penetrate, and generally just not trying very hard to kill an opponent.
Ok, rant aside, I do think it is cool to specifically target limbs and vitals to weaken and subdue opponents, I just think Pathfinder's system of critical hits and such is incompatible with that concept, as it already incorporates a version of it. I am not specifically saying called shots per say. What I am talking about is something like an alternate methodology of hit points and targeting. It is something I came up with many years ago that I never really found a place for, still haven't I guess.
The gist is, take fifty percent of your character's hit points (rounded up), that is how many hit points your torso has. Your head, arms, and legs have ten percent or your total hit points each. Each section of the body has specific effects tied to it that occur at fifty percent, twenty-five percent, and zero hit points, and what occurs on critical hits. This system would replace the various critical feats of the game. Each section of the body would have an alternate AC, with the torso having the normal AC for the character, and the legs and arms, the very mobile extremities, have a dodge bonus to their specific AC, the head would have a natural armor bonus, as skulls are hard.
Is the system any better than called shots? Not really, it is still just a targeting system with special effects based on it. It was just something I came up with a while ago, a different take on called shots and critical hits and that sort of thing. Looking at the called shot rules that recently debuted in Ultimate Combat, I feel that I can be comfortable with using those, though no one has asked that they be included in the game.
So there is some nonsense.