Check out http://paizo.com/pathfinderRPG/prd/magicItems/intelligentItems.html
Specifically, there is a section on "Item Ego" and "Items against Characters" that should answer your questions but what it basically boils down to is: The stronger the weapon (number and strength of enchantments), the stronger the item's ego, the more difficult the Will save. This only comes into play if the character and the magic item are in conflict about something (alignment, personalities, and purpose).
Can any one point me in the right direction. I want to make an intelligent/magic sword in the campaign I'm running. A lot of the magic items in the campaign are rare or unique. Years ago (1E) swords and characters would have ego scores. Usually the sword granted some really great benefits to the character. If for some reason the characters ego score dropped below the ego score of the sword a save of some sort would have to be made. If the character failed the sword could take over and force the character to make bad choices and the like.
Is there anything like that in the pathfinder rules. I can't seem to find it if exist.
Also there is an intelligent item section in the magic item description of the Core Rulebook. This details different options for an intelligent item and how that effects it's "ego" score. Based on the pathfinder version, the ego score is effectively a will save for the character wielding the item.
Slightly OT:
I have a concept for a villian that happens to be an intelligent magic item. Can a sufficiently intelligent magic item be considered to overcome challenges, and can they then earn experience and gain character class levels?