I haven't used intelligent weapons yet IMC, but will
When a character consistently treats a (normal, usually) weapon as something special (Forged it personally; talks to it, as in "Andaris, you were my father's blade. Tonight you shall taste the flesh of his murderer"; or otherwise "makes it feel special" throughtout the course of the campaign), and uses the weapon to accomplishe something significant or extraordinary, it has the chance to be "promoted"
Thus, a 3rd level character who single handedly defeats a creature that's out of his league while using his favorite-to-the point-of-exclusivity battle axe, might find that as a result, his axe has become a +1 weapon.
Later on, as deeds become more heroic, and if he continues using the axe, other qualities appropriate to his deeds might be added (Bane: Giants, etc.), and eventually intelligence.